Using a DLC purchased by another account [duplicate]
Yes, this is possible on a single console with both accounts activated as primary (as you've mentioned). The DLC from both accounts will be usable in the game, and available to the other users on the account too.
You would have to check individually for the game concerned, but the PlayStation Store states that the license allows you to use the DLC and Games on any console where you are activated as primary (which means any other user can use the same content), and one other console you have signed into, but only for your user account on that console.