I'm not sure if it is possible , but i would suspect that they get some kind off penalty in their original class , for diminishing their training efforts and the fact they focus on other things .
Each multiclassing character indeed gets a penalty, of ECL +1 per new class (max 2). This means they own less or just 1 experience from easier monsters, and of course, they are not as rough or potent as a character of their level that has only one class, meaning a wizard/druid/cleric could probably solo some areas on Belinara, but I do know Chakar can't, cause he just does not sport the firepower
Yeah well, it's a game, not RL
When somebody multiclasses, they do not leave their former classes behind. They fuse the training of the old class with the new one, and focus on learning new techniques in the new. They're not JUST learning to be a mage or a fighter, but both, which is why they have the choice between taking a level in one class or the other when they progress.
However, ability depreciation would only happen in a roleplayed sense - and sure, folks who multiclass from fighter to mage and start being a non-combatant, they should think about roleplaying that they've lost their 'edge' in toe to toe battle.