I'm so sorry you're going through this. I was in a similar boat a few months ago, DD is nearly a year old now and I've just under 21 months between her and DS (2.8) - I didn't have PND but I was suffering with extreme anxiety, stress and it turned out I also had complex post traumatic stress disorder as a result of my own crappy childhood. I would like to reassure you that it gets so much better...it happens so slowly that you don't really notice but it does get easier with time.
I've been having hypnotherapy and have taken up meditation - I realised my main 'problem' was I was spending most of the time in 'fight, flight or freeze' - it seemed like EVERYTHINGWASANEMERGENCY ALLOFTHETIME, my stress levels were through the roof, I was pacing the floor, always on the move, always in a high state of alert. it's great that you're getting help from your GP but you can do some things for yourself.
the first thing you need to do like a pp said is to calm down. it's so hard when you're pumped full of adrenaline but what helped me was to root my feet to the floor, breathe in through your nose for a count of 4, blow breath out through your mouth slowly for a count of 12. sit down if you can, I know it's hard when both children are screaming at once and all you can think is 'STOP SCREAMING!!' but it's NOT an emergency, you need to mentally rise above it. when you've done a few breaths like that you should start to feel your heartrate slow down, it should become easier to start to find calm.
honestly I do know how impossible this sounds, but your children need YOU to be grounded, calm and a satellite for them to come back to when their emotions take them somewhere scary. at the moment you are matching their emotional state and there's nowhere safe for anyone to come down to. your emotional state does not need to be this high, there is no panic - they're crying because they need you to guide them, and you can do it - I know it can be done because I was almost exactly where you are now a few months ago. my poor 2-year-old was shouted at more in a few months than I ever wanted to shout at my children in a lifetime 
but it's repairable if you get it sorted now. according to my hypnotherapist children are building their sense of how the world works between the ages of 2 and 7.. now is the time to get yourself sorted, it's not too late.
can you ask your friend to come a bit earlier?
I found a good video on YouTube, I'll try and find a link for you - some nutty woman 
who made so much sense when she talked about a child screaming and all we can think about is making it stop and all our energies go into that when actually there's often nothing we can do to make it stop, she talks of how to stay calm and 'allow' your child to cry safely in your arms. but as others say it's ok to leave them somewhere safe in the house while you take 5 mins to compose yourself
I second getting out of the house, if you've got a sling stick baby in that and toddler in the pushchair wrapped up with a coat over you and baby (raincoat backwards maybe?) and raincover on pushchair and just walk s.l.o.w.l.y - the fresh air will do you all good
I really hope you start to feel better soon and can find some way of feeling calmer right now. ask your friend for practical help (mainly playing with toddler) if you feel you can and if you've a DH be sure to tell him how you're feeling and that you need practical support, and get back to the GP as soon as you can and don't leave until you get referred to someone who can really help - in my case it was a psychiatric nurse who told me it was likely the help I needed wouldn't be covered on the NHS, most hypnotherapists have a sliding scale to make it a little more affordable, it has been worth 100 times what I've paid for it because I've got my life back and my children are better cared for. also check out www.getsomeheadspace.com for meditation - it's worked wonders for me
you don't need to suffer alone 