if he's already running stereo to an effective degree, and you do it, it might make things sound bigger or may muddy it up....it's a fine line.
sonically though, if that spot is already filled then you might just be better off going straight on assault with no effects, in mono....esp if you're the rhythm guitarist...it'll help create more of a backbone and lock you in with the drumz and bass.
to answer your question though, the big sound difference with 2 guitarists running both in stereo is dependant on the music being played and whether or not you both or either use a lot of effects.
stereo sound with chorus and some intricate delay programs can sound incredible up close and on stage. FOH, if you're not each mic'd stereo and getting 2 channel per guitar at the board, and being mixed properly (which, unless you have your own soundguy, you're probably going to get one channel each so it defeats the purpose), stereo stage rigs (that output different sonic signals) can actually throw off the front of house sound and possibly create some phasing issues.
i've tried a TON of different approaches with stereo rigs, stereo settings, etc, blah blah blah...and live, it always was general consensus from my bands and soundguys i worked with that a mono signal always cut thru best... YMMV obviously, but, you get into some weird shit with stereo rigs pertaining to Front Of House sound...esp if you've got a guy that's an asshole and doesn't want to work with you at all, and barely wants to mix your band to begin with.
that being said, i've played a TON of small club gigs where the guitars weren't even mic'd up at all, and a stereo rig REALLY helped fill out the room, but, bear in mind, a true stereo sound will spread out nicely for guitar, but once the room starts filling with people, you'll probably find yourself turning up a bit to compensate.
my other input regarding this is, if you can get your cabinets up off the floor (on some milkcrates or a chair) do it...it'll help project a stereo sound a bit better than being on the floor and sounding so muffled.
good luck!!!!