2 sole props are a common control group - deductions?

[Belgarath]

Here's an interesting situation.

You have two spouses, each of whom have their own sole proprietorship business. NO other employees. Spouse A who makes scads of money is setting up a DB plan. They are a Controlled Group, or I guess technically a Common Control Group, which has the same effect. Due to the minimum participation rules, spouse B who makes very little money must be included in the plan, since there are only the two of them.

We're having a discussion - one seems to be telling us that some of the costs for the benefit of one spouse must be allocated to the other – rather like a 50/50 partnership. I don’t think this is necessarily required – it seems like it should be two entirely separate calculations – one for each sole prop, based solely upon their own Schedule C income, as they are separate entities, even though part of a controlled group.

Am I missing something here? Are you aware of anything saying that two such spousal sole props in a controlled group are treated as a partnership for cost/deduction purposes? I know there is “joint and several liability” under IRC 412(b)(2), but in the absence of any regulations, wouldn’t it be permissible/reasonable to simply allocate as per their own calculation based upon their own individual Schedule C’s?

Alternatively, is it reasonable for her to pay the entire contribution, as there is joint and several liability?