All IRA balances are aggregated to calculate the RMD, and the money may be withdrawn from any of the accounts or any combination. Remember also, an RMD is not a specific, extra transaction, it is the amount you must withdraw over the year to satisfy the IRS. For example, if your RMD is $5000 and you have already been withdrawing $500 per month for living expenses, you have already met your requirement for the year.
However, if you have money left in qualified workplace plans (401k, 403b, etc.), their RMDs are calculated separately from your IRA RMD.
Also note that Roth IRAs don't have an RMD and the balance of a Roth IRA is not included with your pre-tax IRAs when calculating your RMD.
