I don't have personal experience in this area, but this seems logical: If you take the stronghold for your first class, then the game registers that you already have a stronghold, you can only have one, so thats it, its over. However, if you tell the first person you will think about it, then the game does not record that you have a stronghold, but it does record that that person is waiting for a reply. Move on to the second stronghold, when you try to get it the game searches and finds that you don't have a stronghold, thus you can take this one since it is one of your classes. Now the game registers that you have a stronghold, but since the designers never thought to put in a line for the first stronghold character like "sorry you already have another base now, I retract my offer", if they are waiting for your reply, you can always go back and say yes, earning you a second stronghold. I am pretty sure that is how the game thinks, though I could be dead wrong, but it just seems logical to me.
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"Not everything ends the way you think it should"