Hi all, thanks for your advice and comments so far.
Using an online Speaker Ohm Calculator it tells me that connecting both speakers in parallel at 2.67 ohms will distribute the power 33.3% to the 8 ohm cab and 66.7% to the 4 ohm cab. Given the amp outputs 700w @ 2 ohms then this would equate to 175w to the 8 ohm cab and 350w to the 4 ohm cab.
Given the 8 ohm cab can handle 400w and the 4 ohm cab 800w I think it would be best to output to each cab from each channel as that would give me 280w to the 8 ohm cab and 450w to the 4 ohm cab. That’s 730w in total rather than 525w. Plus using this method means that if one of the amp channels went down, or one of the cabs for that matter, then it’s less hassle.
Hopefully my conclusion makes sense, but if I’ve got anything wrong do let me know.
Thanks again, Jamie