Dual class can get either stronghold, same as multi.
There is a glitch which allows dual and multi classes to get more than one stronghold, but I'm not sure what it is - I think if you are a fighter/thief for example, you would clear out the keep but not speak to Nalia afterwards, go and get the thief stronghold, then go back and speak to Nalia (or something like that).
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas