why are two subs harder to place than full range?

sorry if this is a dumb question, but i notice that whenever someone talks about getting stereo subs there usually ensues much hand wringing over placement and room modes, etc., but this rarely is the case when someone talks about full range speakers with large 10 or 12 inch woofers (with or without built in amps for them).

Is there a sound technical basis for this different treatment or is it merely convenience, i.e., since subs can be moved about separately from main speakers, then it makes more since to talk about eliminating room modes by moving them around?

I'm getting a second sub for a rather large vaulted room, have an injured back, and would rather minimize what seems like a rather long process, at least for the time being by just keeping the subs with the mains.

thanks