Honestly, don't do it. There is no real advantage to doing so unless you REALLY wanna be a different class. If you must, do it early on, like lv 7 or 10.

You don't lose your spells. If you have the pack space you can be all three classes. It is only a matter of which gear you currently have equipped.

As for advantage, each class is different. I prefer zerk for PvE, and class advantage for PvP (zerk over pali, pali over witcher, witcher over zerk).

To Expand on Trongster's point:

Dual classes are a matter of capriciousness. You can function just fine with one class: THe best players are only one class. However, elite players have devoted time and resources to get purple gear for 2 classes, which is extremely hard. Unless this is your idea of fun and you're willing to plunk down much more cash/time to do it, don't.

Marcus is Wise. I started going multi-class only once capped at level 50 and making gold without getting experience. If you try to build dual-class in mid-level, you'll likely level too quickly, neglect other aspects of the game due to gold consumed on gear, and not play either class to its fullest.

If you want to switch, switch sooner rather than later. Once you go purple gear it's non-transfer so there's no reselling. Pick your class by level 31 and legionnaire.