You'll need to find out how ohms each speaker has in each cabinet and how they're wired.
Series: R = R1 + R2 + ... + Rn ohms
Parallel: R = (R1 x R2) / (R1 + R2) for 2 speakers; or for more than 2 speakers 1/R = 1/R1 + 1/R2 + ... + 1/Rn
(i) Two 4 ohm speakers wired in series gives you 8 ohms.
(ii) Two 8 ohm speakers wired in parallel gives you 4 ohms.
(iii) If you were to wire in parallel 2 sets of two 8 ohm speakers (which are wired in series), that'll give you 8 ohms altogether:
Ra = 8 + 8 = 16, Rb = 8 + 8 = 16
R = (Ra x Rb) / (Ra + Rb) = (16 x 16) / (16 + 16) = 256 / 32 = 8 ohms
Sounds to me like the speakers in each cab may have a different rating. You would need to look inside your marshall cab and determine how many ohms each speaker has and how they're wired up to see if you can rewire it to give the cab a 16 ohm rating. You'll not get a balanced sound otherwise when using all the speakers of both cabs, no matter what you set your amp to.
Best that you speak to / have the work carried out by a professional to make sure that you don't break anything.
Better yet, you could try and and find a different cab rated at 8 ohms or trade your marshall cab for a another peavey cab.