Cats are avoiding the carpet

Getting rid of fleas is HARD once they've gotten a foothold. I do have some thoughts on your flea situation, and the cats' behavior.

Since both cats seem to be avoiding the carpet, it seems likely that you do still have an infestation going. You can monitor the number of fleas in the home by placing a flea trap; either purchased, or make your own. (Place a small bright lamp over a dish filled with an inch or two of water and a squirt of dish soap. Leave the lamp on overnight in an otherwise dark, carpeted room, and in the morning check if any fleas have been trapped in the water.). These traps aren't effective enough to remove the infestation alone, but will allow you to see if fleas are present.

To really get rid of fleas, you may have to have a professional exterminator in. You will also need to be exceptionally diligent, vacuuming at least daily (more than once a day is better, dispose of the contents of the vacuum bag outdoors!), washing the cat's bedding and preferred resting spots often and treating the cats for fleas reliably for several months. If you let up before every little egg is gone, they will come back!

I recommend you talk to your vet about giving the cats daily, or every 2-3 days Capstar for a while, and maybe see if you can shut your couch-pottying cat in an uncarpeted area for now. While cats don't like to be confined, the behavior is a clear sign of distress, and the more times the cat eliminates on the couch, the more of a habit it's going to be to use furniture as a bathroom. It may be an ongoing problem even once the fleas are resolved if it becomes the cat's preferred way to 'go'.