#1
After some advice. I've got a Marshall mc412c it's got two inputs stereo and mono. I've got to play a gig at a very small venue. Would it be possible to plug two heads into this cab or will this damage the cab?
#2
I find it pretty comical that you're thinking about using 2 amps when you just said it was a tiny venue.

As with anything regarding the way amps and cabinets are wired (because they're not all wired the same way), RTFM.

Not sure who recommended you buy an MG412 cab though. But they're probably deaf as a lamppost.
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#3
Flip your cab to the stereo setting and pay attention to the ohms rating for the 2 heads.

Are you guys sharing the same cab?
Guitars:
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Amps:
Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .
#4
For a while I worked with two stereo 2x12s, one on either side of the stage (well, either side of the drummer plus a bit). One of us took both right sides, the other both left sides. It allowed us to hear what the other was doing and what the balance was. At another point, I put guitar through the right side, keys through the left side. Again, it was so that everyone could hear wherever they were on the stage, and it was pretty effective when we didn't have room for monitors, etc. and for when the PA was vocals-only.
#6
As diabolical said, you have to re-wire the cab internally. You can't just plug two amps into one cab - something bad may and will go wrong (Murphys Law).
If it's something that you want to do often, ie. use one amp with the 4x12 and then for another song use a different amp with the 412, then there are devices where you can plug both amps into, which then leads to the amp. Check out the TINO System from Palmer (Google it) for example. You can then switch between the two amps.
Otherwise, have someone mod your cabinet to swith a 4x12 to a 2x12.
Hooper drives the boat chief...
#7
Quote by jackmiddlemiss
As diabolical said, you have to re-wire the cab internally. You can't just plug two amps into one cab - something bad may and will go wrong (Murphys Law).
If it's something that you want to do often, ie. use one amp with the 4x12 and then for another song use a different amp with the 412, then there are devices where you can plug both amps into, which then leads to the amp. Check out the TINO System from Palmer (Google it) for example. You can then switch between the two amps.
Otherwise, have someone mod your cabinet to swith a 4x12 to a 2x12.

You don't need to rewire anything if the cab is already wired for stereo/mono, like TS already has.  

On topic:
Set the cab to stereo.  The only thing I can find for that cab is that it is 200 watts I can't find a pic of the jack pannell for specs on ohms for each setting.  It should have all the info you need on it.  When you hook 2 amps up to it just do not exceed the ohms for each jack for stereo mode and do not exceed 100 watts on each side for each amp.  So you could run 2 amps that are 100 watts or less each and just match the ohms of the amps to the ohms of the cab in stereo mode.  The amp is already wired to accommodate 2 mono amps or 1 stereo amp.  The only thing you may want/need to change is adding a separation barrier to isolate the 2 sets of speakers from each other if one isn't there already for best separation.  That way the one set of speakers do not affect the other set while playing different things.