You must keep some separate. Here is the IRS rule:
An IRA owner must calculate the RMD separately for each IRA that he
or she owns, but can withdraw the total amount from one or more of the
IRAs. Similarly, a 403(b) contract owner must calculate the RMD
separately for each 403(b) contract that he or she owns, but can take
the total amount from one or more of the 403(b) contracts.
However, RMDs required from other types of retirement plans, such as
401(k) and 457(b) plans have to be taken separately from each of those
plan accounts.
https://www.irs.gov/retirement-plans/retirement-plans-faqs-regarding-required-minimum-distributions#...
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
