[PyroDrew]

Like George said hex-editing can get around this restriction, but if you don't want to go down that route...

You can take 2 different class paths from any 2 different guilds. Thus if your 1st class was Mage & your 2nd class Rogue you can take your Mage line from Mage to Sorceror to Wizard. To complete your character you can take Rogue to Trickster. It doesn't matter whether you do Trickster or Sorceror first. From what I can tell your character cannot take 2 paths in the same class line (Mage, Adept, Rogue, Fighter lines). So you cannot be a Wizard/Battlemage/Fighter or a Wizard/Shugenja/Adept for example.

If you're still confused there is a character planner somewhere here (which the search engine should find), which is good to show you what is class combos are possible... but I found it's information regarding what skills associated with those builds to be outdated.