Beneficiary specified in SA?

My spouse wants the sep agreement to indicate that each of us will keep the other as beneficiary on all investment and retirement accounts, for life. I believe this would require a pre-nup in the event of a re-marriage because i believe by law the spouse is the beneficiary on a retirement account unless they sign a waver. But is what my spouse is asking for even something that could go into a sep agreement and would it be enforcable?

Yes, it would be enforceable and your new spouse would not be entitled to receive benefits in the event of your death.