Actually, the RMD for each IRA is calculated separately, then aggregated. The general result for satisfying RMDs is the same, but it makes a difference if, say, you want to satisfy the RMD for one of the traditional IRAs and then do a Roth conversion from that IRA before satisfying the RMDs for the other IRAs. The RMD for a particular IRA must be satisfied before any other amounts distributed from that IRA are eligible for rollover.
IRA RMDs can be aggregated with other IRA RMDs and 403(b) RMDs can aggregated with other 403(b) RMDs, but RMDs for other types of accounts are not permitted to be aggregated.
