In a community property state, husband and wife own 50% each of S Corp with a SIMPLE plan. Both are officers, and receive wages with SIMPLE participation. Wife also has unrelated Sch C income and wants to make maximum SEP contribution. Is this OK, or by attribution rules does she own > 50% of Sub S so benefits must be equal in both Sub S and Sch C?

Although it may have serious side effects (e.g, if there is a divorce or determining basis if death occurs, and it will), they may be able to have their stock treated as separate property. You need to get a copy of Derrin Watson's Book, Who's the Employer?--see chapter 7 (especially Q 7:17).

Derrin's Book

I don’t have my copy with me, but I assume that section of the book refers to the exception under § 1563

Or course, Derrin's book explains it in 'English'