I can play like this for an hour or so
Get a multi elevation desk (around 20 bucks from office depot) it allows you to lower and tilt the TV UP toward you reducing neck strain
adding cheap armrest bars/aerobars (also around 20 bucks on Amazon) allows you to rest your elbows comfortably while riding the bike, keeping your weight forward.
Adding a heart rate momitor that beeps when you slow down is a good way to remind yourself to speed up.
Tips
Sweat will be a pain with the controller so keep a towel handy.
My Trainer is rather loud, so I use (in the ear) - ear buds. I originally used a cloth - full headset, but the sweat made them stank up.
Doing both tends to create a seesaw effect - Games that require alot of concentration will naturally make you slow down. Going faster (higher heart rate) tends to increase the games difficulty.
As mentioned by @AndyP below, this will likely decrease performance (at least initially) in both gaming and cycling while doing both, so I wouldn't recommend using this as a method to increase performance in cycling or gaming.
With that said - it is a decent way to burn excess calories and build muscles while performing an otherwise stagnant activity.