Okay. If either plan is integrated you will probably need software, available on BenefitsLink at:
http://www.benefitslink.com/GSL/QPSEP_profile.html
to crunch the number unless you know what the percentage going to the non-owner(s)is. If you're trying to give owner a specified amount or percentage (eg 25%) then you don't know what the formula is that gets him/her there; hence you don't know what the percentage is for nonowners (and can't compute SE tax).
Once you know the nonowner amount/percentage, then you can figure owner's SE tax (if there are g/l from unrelated entities or any W-2 income that needs to be taken into account for SE). Then subtract 1/2 of it from owner's pre-plan income (along with the nonowner contribution), then use equivalency percentage (25 = 20 etc) to arrive at owner's contribution. To start, treat both plans as one (add formulas together, even if integrated). When you get ultra net EI, then apply it to the MP plan formula exactly as the formula reads, what's left goes into the SEP. It is easier with software (and is handles the same way, except that the math is instantaneous and you don't have to compute SE tax; it's done on the fly)!!