Yes, no problem at all.
This is the "buddy" seat in 1A on the A380 - it has a seatbelt in case it is required during turbulence.
If anything I think the buddy seat may be slightly wider on other seats in the cabin and in the middle seats, there is a slight curve at that part of the airframe which makes the overhead locker slightly less spacious too. I'll see if I can find a photo.

Edit: a middle seat buddy on the A380, don't think it is any bigger actually:


Edit 2: The other one you can buddy dine on is the old 777 F aircraft (i.e. not the refurbished 77Ws), with the screen stowed obviously.
It's narrower than the A380 though.