Originally Posted by surfinusa
they are usually lazy and sit in the corner of the tank
This is not necessarily true. While they are bottom dwellers and in the wild they are mostly nocturnal, in your tank they will become more active during the general feeding times. You can expect them to be very active for at least a couple hours before and after feeding.
As far as your other question; whether 2 will work in a 400, you will probably hear several opinions on this. As long as the dimensions are a minimum of 8' x 3' (which I'm sure they are) then I'd say it's fine. Just be careful with the aquascaping... enough to make caves but also enough swimming room.