The main problems running two amps into a single cab are:
1. The output power of 2 amps combined could easily blow the speakers.
2. Both amps have to be outputting to the speakers at all times or else one amp will blow (its transformer).
3. You won't get the sound you're looking for because instead of each amp driving the speaker differently and the sounds then interacting, the speaker will be driven by the combined sound of both amps (possibly fixable using a multi speaker cab and hooking up separate speakers to separate heads).
Hence 2 amp setups usually have a dedicated cab for each head.
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