An RMD is not a specific transaction. It is the minimum amount you musts withdraw, sometime during the year, based on your account balances and your age. You can always withdraw more if you like, but you will be penalized if you withdraw less than the minimum amount.
The minimum amount you must withdraw from your IRAs is based on the combined balance of all your IRAs, and you can withdraw that money from any of your IRA accounts in any combination, as long as the total of the withdrawals is equal or more than the minimum you are required to withdraw.
Don't confuse IRAs with other types of retirement plans like 401(k)s. An IRA is not a 401(k) even though they have similar purposes--they are controlled by different sections of tax law. If you have an IRA and a 401(k) (or other workplace plan) those RMDs must be calculated and taken separately.
