"I don't see any reference to married filing separately there?"

Question 58 is for Social Security, Railroad Retirement and Department of Veteran Affairs benefit recipients. That's what you asked about.

First, it tells you there is Nothing needed to be done for the client, the Beneficiary, as you describe. The client that gets SS direct deposit would get the stimulus DD as well; otherwise, as a paper check.

Second, it explains how filers will get their payment, which is the scenario for the spouse you describe.

They are both explained there.