There was a gadget you could get in the days of the PowerPC minis that would let you plug two monitors into it—it faked out the mini somehow. I’m not sure how well they worked, or if you can still get one now.
Matrox make a non-USB adapter that is quite often cited for this use and is compatible with PC and Mac – it spoofs the resolution of both monitors into one big monitor the Mac detects, if I’m not mistaken.