IRA owners can clearly combine the accounts they own and they can combine the required minimum distributions (RMDs) from multiple IRAs and take them from any one or combination of their IRAs. The rules for combining Inherited IRAs and RMDs are more complex.
An IRA owner cannot combine IRAs they own with IRAs that they have inherited, unless the inherited IRA came from their current spouse. IRAs that are inherited from the same person can be combined, as long as the RMD calculation is done in the manner for all of the inherited IRA accounts. Generally, this is easy. If Dad had two IRA accounts and you inherit half of each of those accounts because you are named on the beneficiary forms for those accounts, then you can combine them. If you keep the accounts separate, you can calculate the RMD on each account and then combine it and take all or any part of the RMD from either account as long as you take the full RMD.