the only time I saw anyone dual class was when we jumped too many levels early as thives.
I jumped from level 1-8 in theif one night becuse we stole a boat that ended up having thousands of gold worth of goods on it. We did out the xp (2xp per gp) and I jumped 7 levels, so I quickly dual classed into wizard, and found my self in a 2nd level game with 8d6 hps as a 1st level wizard. I spec in Evoc and when I hit level 9 picked back up my shortsword and leather armor...
I think here is where the no more than one level per adventure rule would have been really handy (well, it was that you couldn't gain more XP until you did training but it is the same thing).