Unfortunately, you cannot, as the combined accounts must be of the same type.
A single RMD withdrawal can be made to cover the entire required amount for the year. However, the combined accounts must be of the same type. For example, two traditional IRAs can be combined, not an IRA account and a pension account.
If the accounts are combined, a taxpayer may take a withdrawal from any chosen account in the amount of the total required minimum distribution required for all accounts.