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Depending on your reasons for wanting to combine the miles there may be a workaround. If you have enough miles in each account to redeem for an award and are trying to get two seats you should be able to book both seats on the same flights using the two different accounts. Say you want to fly from JFK-LAS. If Delta has 2 seats available you can redeem one using your NW miles and one using your DL miles, both on the same DL flight. Of course, if you have 10K in one account and 15K in the other and are trying to combine them for a booking that won't work out.
And going forward you should pick one carrier within the alliance and credit everything to them. That will help out in the future.
Finally, if the rumored NW/DL merger ever goes through your accounts will likely end up merged, but with the way the pilots are playing that will take a while.