Sorry, just previewed and this is a bit of a mega post, but we have been dealing with something similar, so I thought I might be able to offer a little advice.
First off, you have my sympathy. My lurcher recently developed separation anxiety after the death of our older dog and we found out via a rather rude post-it note stuck to our front-door. (Ridiculous thing being, I am never out for longer than 20 minutes at a time, but just by coincidence that week the poor dog had been an inpatient for a procedure at the vets for the first time ever and the following two days I had two appointments - both of which meant I was out for just under an hour.)
In the first instance, sit and have a think about what you do to prepare yourself for going out for the day, on days when you are working or going to be out for some time.
You might for example, wear a particular coat or pair of shoes, pick up a certain handbag, do your hair in a certain way and/or check your appearance in the mirror the second before you leave.
You probably don't do those things when you are just popping onto the drive to shuffle cars around or do the bins and in fact might not consciously notice that you always do the same things, but your dog has almost definitely noticed and will therefore be starting to get anxious before you leave - hence knowing the difference between you moving the car and going to work.
You need to both de-sensitise your dog to those pre-departure signals and counter-condition your leaving to mean something good is going to happen, rather than just you leaving (which obviously is bad).
To de-sensitise your dog/s you need to do all your usual pre-departure routines, but then not leave. To start with you can just select elements of the routine - at first put your coat on - then sit down on the sofa and watch tv or read a book, then gradually add in more elements, mix them up and each time don't leave the house, but instead hang around, have a coffee, read a book, watch some tv or something.
When you have reached the point that you can do the whole routine in order without your dog showing any signs of anxiety, you can start to approach the door you leave through, but again don't actually leave. With some highly anxious dogs this might involve gradually approaching the door one step at a time, only moving another step when you are sure the dog isn't showing signs of being anxious. Next you would touch the handle, then open the door, then stand in the doorway, then step through the doorway, then close the door for a second and reopen it immediately, then start building up the length of time you leave it closed for, maybe 30 seconds, then 45, then 60 then 90 then 2 minutes then build it up in 1-2 minute chunks until and so on and so on. Once your dog starts to relax, you can build up the length of time you leave the door shut in larger chunks.
Next you would gradually start stepping out of the front door, then stand on the doorstep, then step off the step and gradually build up to walking down the drive etc. Then you may need to start deconditioning the sound of you getting in the car (at first just opening and closing the car door without getting in, then getting in for a second, then building up the time between the door closing and engine starting and so on until you can get in, start the engine and have built up to pulling off the drive). Your neighbours will start to suspect you are a little bonkers - but who cares!
You need to take each stage very slowly and gauge your dog's reaction. If at any point he/she starts to look anxious, you need to go back a stage and slow down the process. Each stage of the de-sensitisation will probably need to be carried out at random times, as many times a day as you can manage whilst retaining your sanity.
It's more difficult if you have two loose dogs to counter-condition the anxiety he/she feels when you leave for a positive feeling. With my dog, I started giving him a treat toy (stuffed kong or treat dispensing ball etc) whenever I did my departure routine. Each time removing the toy as soon as I returned - so if I only stepped out of the room for a second, I took the toy away as soon as I came back in. This is to reinforce the fact that he only gets the lovely tasty toy when I go out and ultimately get him to look forward to me leaving so he can have a good scoff. 
(Just remembered, it's a good idea not to greet your dogs when you return, but wait until they are calmly settled - either lying quietly or ignoring you somehow - before you approach to say hello. It seems mean - but it really does help them to understand that you leaving and coming back isn't such a big deal. I actually leave the kitchen door closed for a good 5 minutes when I return now, so that my boy knows I'm there, but I'm not desperate to get to him, so the fact that I went out can't be that big a deal, iyswim?)
If your dogs are likely to fight over kongs etc then that will make things harder if you can't separate them - at least by a baby gate or something. Not sure what to suggest in your situation, but perhaps you could leave just a small treat in each dog's bed so that as soon as you leave they find it and associate that finding with you leaving or something?
We are now 3-4 weeks-ish in and my boy can't wait to get his treat toys. As soon as I start organising myself to leave he goes and waits by the kitchen worktop that he knows his treats are stored on and when I put them down he starts noshing without so much as a backwards glance.
I did have to teach him how to unstuff a kong, but then - being a lurcher - he's not the brightest bulb in the box.
I record him using a handheld dictaphone (I think we bought it from Argos) so I know what he's up to while I'm out and when we started all this he began howling and barking about 4 minutes after I left. Now I can leave him between 20 minutes and half an hour while he quietly plays with his treat toys and he no longer swamps the kitchen in drool, but is waiting in his bed looking calm when I return. We have had three relapses where he's howled the place down, but each time I think I have identified a trigger and been able to rectify it. I have to leave him 3/4 times a day every day for school runs that last anything between 10 and 20 minutes, so I feel that's real progress.
Dealing with separation anxiety can be a really slow and frustrating process, is often three steps forwards, two steps back and is made even harder if you have no choice but to go to work and leave the dog during the process. Barking and howling itself is self-rewarding, as it lowers stress levels and releases endorphins which encourage the dog to continue, so you are constantly working against the dog's own chemistry. If you stick with it though and do as much desensitisation and counter-conditioning work as you can when you are home you should see some improvement fairly quickly.
Good luck.