I didn't think it was possible, but I have two. I dual-
classed a berserker into a thief, as soon as I got out of
my cage. I'm in Chapter 2 now, I've taken control of the
Thieves Guild from Mae'Var..RIP...I've already had a couple
of my "workers" tossed in the Greybar motel. I finally
decided to go help Nalia with her castle. After clearing it
out she gave me the option to run it.
I thought maybe it would have messed up the Thieves Guild,
but I just went back and I'm still getting "Nothing further
to report." messages. So I'm assuming that I have control of
both.
Is this normal, or will I lose one or both?

Yeah its possible if everything is done in the correct order that a dual classed character can have 2 strongholds. I don't think it was intended to work like this but it does...

No complaints. It's giving me a little extra cash before I
hit the Underdark. Too bad the poor knights keep warping
in to get stomped, though.