That's multi-classing. Dual classing was different. You would actually change classes. For instance you could be fighter level 4 and rouge level 16 for 20 total level. One you switch to rouge though you'd only have your HP from being a fighter. Once you reached level for you'd have access to all your fighter abilities.
I used it a lot in nwn, especially after DLC allowed you to go to level 40 I believe.
I'm familiar with dual-classing, but that was in 2nd Edition. ToEE uses the 3.5E ruleset.