Since there will be some possibly significant differences in the distance between inidividual instruments/sound sources on stage (*see below), I'd be afraid of phasing issues between the two different m/s rig/locations unless you decode/mix each one to mono first and then use the two mono sources as the left and right channels of an ultimate stereo mix. It's worth a try, but I'd agree with SmokinJoe's suggestion that you're probably going to be better off running the 414s as an m/s pair in the center of the stage lip and using your 480s as flanking mics (ideally w/ omni capsules if you can borrow some).
*
_______ ____________ ___________ _________ ____________
GTR AMP BASS AMP DRUMS GTR AMP KEYS/HORNS
M/S PAIR 1 M/S PAIR 2
______________________________________________________________________________
STAGE LIP
In the scenario above, for instance, the difference in time between when the stage left and stage right guitar amps hit each of the two m/s pairs may result in significant delay differences that are big enough to be audible. As such, a discreet stereo mix from each of the two m/s pairs will have very different sound staging/spacial imaging compared to the other. When overlaying those stereo images, I believe the result will be out of phase for some things due to these spatial differences. If you mix each source to mono, however, that should eliminate the potential for one or another channels of each of the two sources from being out of phase with the corresponding channel of the other source. However, even if mixing the two disparate m/s pairs two mono, the left mono mix and right mono mix still may be out of phase with each other - but, at least you will not be compounding that by adding potential out of phase issues between corresponding 'left' and 'right' channels if you were to try decoding each m/s pair in stereo before mixing them together.
I think the following set-up produces results that will more consistently allow a stereo mix with minimized phase issues:
_______ ____________ ___________ _________ ____________
GTR AMP BASS AMP DRUMS GTR AMP KEYS/HORNS
LEFT FLANK M/S PAIR RIGHT FLANK
______________________________________________________________________________
STAGE LIP
Here's an example of the m/s center pair + flanking omnis option:
http://www.archive.org/details/gm2007-05-18.akg414midside_452ck22.flac16