The trouble is once you have two it is a short step to three!

things to think about - double the costs - be realistic - two lots of dog insurance, two lots of vaccinations, wormings, food, collars, dogwalker if used, leads, dog beds, etc. I'd suggest doing a spreadsheet of what you spend at the moment, and this will help you see how it fits with the family finance. Don't forget also if you go on holiday without the dog, that's two lots of kennel fees. Gulp.
Transport - depending on the size of your car, will it still be appropriate for two dogs? We moved up from a Ford Fiesta when we had one, to an estate car.
Agree about the cleaning!! Definately more mess.
If you have someone else who pops in and helps out, say if you are out for the day, sometimes it can be harder for a non doggy neighbour to manage/cope with two than one. Again, depends on yr circumstances.
As for getting on, prob a good idea to get a dog if you already have a bitch, but really it depends on the dogs concerned. We've done fostering and some dogs have been World War 3 IMMEDIATELY to the extent of finding another foster home, while others have slotted in as though they have always known us - virtually impossible to predict. Best way is to take your own dog with you to do some introductions and see how they get on - do a walk together - often good to go for a stroll on "neutral" territory - so you can assess how they are getting on. It's like running the UN sometimes.
Most rescues and reputable places will insist on this anyway.
Have listed all the negatives. Positives, your dog will love having another dog around to play with and socialise with. They are not that much more work and so much more happiness. You may find you need an extra sofa! Um, really, I think if you are truly into dogs, you almost invariably end up with more than one. I dont' regret it for a second and am constantly telling DH we could probably squeeze in a fourth ....
Also, I'd make sure that all the adults in the house are completely signed on to the idea - not fair on anyone otherwise.
Good luck and keep us posted.