[Radek]

If you are playing ToB then dual at level 13 because of additional benefits gained by any fighter class at level 13 (+1/2 attack, etc.)
If you are playing a plain SoA then level 12 is too late (except for soloists, duos and perhaps trios), even if you are dualling to a thief or a priest. You have spent 900 000 XP already. You will need next ca. 900 000 XP to become a dual class. It's too much. Also, we can see that dualling at level 13 in SoA is pure nonsense (again, except soloists etc.) You would become a dual class in the Hell most likely.