The mismatched drivers thing can be like pissing in the dark; you won't know unless you try. Could work, could be awful. But, you will be able to drive each speaker with its own independent amplifier, which means you can EQ them differently and then potentially avoid some (not all) of the pitfalls of mismatched drivers.
A better plan would be to keep them both and then get an extension cab for each (matched to each), then you have LOADS of options