Chances are one of the cats urinated on the sofa because of stress due to the new arrival. Change makes cats stressed, but in due time they usually get accustomed to whatever the change is. Give your old cat lots of love and attention so he knows that he's still part of the family.
In regards to the couch, make sure you clean it right away. I have found the best product (I've tried many) is nok-out. It is safe to use in the washing machine for your cushion cover. Use it in a spray bottle to soak the foam pillow. It's important to remove the scent completely, as the cat(s) will return to the urine scent and do it again. Regular soap will not work! I've heard vinegar and water works as well, but it is a bit smelly. Some cat owners say the enzymatic cleansers work, but I haven't had great results.
Next, you have to keep the cats off the couch for awhile when you aren't in the room to prevent future incidents. I bought a sofa scram matt on the internet, that works wonderfully well. It lets off a high pitched beep when the cats sit on it, which is unpleasant but not harmful to them. If you aren't frustrated enough to buy one, I've heard that the bumpy underside of a rubber welcome matt works as a surface deterent. Some say tin foil works. This did not work at all for my cats. They actually liked it and started chewing at it (dangerous).
My local pet store has a product that I have not yet tried, but looked promising. I can't remember it's name but the premise is that it's a can of compressed air attached to a motion detector. When the cats pass the area you don't want them to be, the device lets off a strong burst of air directed at the cat.
One more alternative, that may also de-stress your cats, is a plug-in feline hormone diffuser. This you can get at the vet and some pet stores. It's pricey, but some swear by it to calm their kitties and eliminate bad urination behaviour.
At the very least, try a shower curtain. I tried this. It saved my couch, but they continued to pee on the curtain. Plus the plastic curtain was ugly!
It is worth checking with your vet to eliminate the possibility of a urinary infection, common to older male cats.
Good luck. Hopefully, your cat pee problem was a one-time-only event.