I think it's fair to say that using 2 or more subs is generally better than using one...Especially in a Theater setting. In my set up I use 2 JL 10 subs. Theater has some dramatic, explosive LFE information. I find using one sub is not as even and you can localize the sub that is your brain knows where the impact is coming from...BUT with both subs engaged there is no localization and it's like a wave of smooth bass.
The bigger speaker will probably dip a little lower in freq response...
I use an Rel R-305 in my 2 channel that just ever so slightly underpins the front speakers - it is really barely on and only engages at about 28 htz...However, I have tried that sub in a theater setting and it can not produce the wallop without distorting..Just an FYI.