TIAA told me that the IRS permits RMD aggregation from multiple 403(b)s, but their system can't do it as a single aggregated RMD - thus I had to do one RMD plus a cash withdrawal to cover the RMD for the other account. (And then they told me that they are not allowed to give tax advice, so they can't tell me how I should report it.)
I've added more information about this as the last section of the main question.
