I always want to dual after level 9, to ensure max hitpoints are achieved. Other factors to consider:
1) Did you install the exp cap remover? For example, for fighter/mages, I prefer to dual at levels such as 13 and above since I am no longer restricted by the exp cap. Hence I get a character who have more weapon slot proficiencies and can also be a great mage.
2) Anticipation of the expansion. With the upcoming expansion, it is likely the exp cap will be increased. Similar concept to #1.
3) If you do not plan to install the exp cap remover, than make sure you use the character development chart well provided in the manual. Keep in mind the 1st class's benefits will not come back until the 2nd class is at least 1 level higher than the first class. Try not to waste exp points (remember you only have 2,950,000), optimize between the two classes so that the "extra" unused exp points for the 2nd class is minimized.
4) Composition of your party, (again I am using a fighter/mage as example), if you already have 2 or more fighters in the party, it will be a good idea to dual the fighter at earlier stage/level (e.g. level 9)since your party will probably benefit more from a mage. For a party with 2 or more mages, you might want to dual class at later level (e.g. level 13)since a more tank-like character might be appropriate. As many others have side, "balance" is the key.
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