If you parallel the two cabinets, the resulting nominal impedance will be around 5.3 ohms. This uneven load distribution not ideal; the lower ohmage Marshall cabinet will draw something like two thirds of the power. So on the grounds of power alone, it may be louder by several decibels, and if the speakers in the Marshall also happen to be more sensitive than the Peavey ones, the Peavey cabinet might as well not be there. Do an experiment to see. (Use the 4 ohm setting on the Marshall amp.)
It could work out as follow: suppose the quieter cabinet is close to you, functioning as a monitor, and the louder one farther way to give you a more ambient, somewhat delayed sound, resulting in an interesting fuller overall sound.
Cabinets cannot be rewired such that their impedance is changed by a factor of two, only by a factor of four, so they cannot be made to have equal impedance. However, the possibility exists of swapping pair of speakers between the two cabinets. Then you can make two equal-impedance mongrel cabinets with an identical configuration of speakers: perhaps two 24 ohm units that can be paralleled to make a 12 ohm load.
All in all, odd numbers of speakers, or imbalanced speaker impedances, are not such a great idea.
Solid-State Guitar Amplifiers by Teemu Teemu Kyttälä, section 11.17 "Typical speaker wiring configurations".