I’m a fighter/berserker and I want to duel class into a cleric in BG2, is the best time after level 9 and how does it work? I lose my proficiencies but I can reallocate them into skills that I already have?
When you dual class, you lose all your class abilities until you gain more levels in your new class. So the time to switch depends on what class abilities you want to have in the original class, traded off against how much "downtime" you will have in not having access to your original class abilities and what level the XP cap will allow you to get in your new class.
Often fighters want to go after level 7 or 13 due to multiple attacks. Kensais sometimes want level 9 due to kit feature intervals. I'm not sure if berserker has anything that makes level 9 better than level 7 though other than that's the last level that you get full fighter d10 + con hp, and for the extra 2 points of THAC0.
Generally speaking you should not put any proficiency points into weapons you already have proficiency in because they don't add up, they'll be replaced by your original pips (if greater) when you regain your class features. Otherwise, you're limited to the choices normally available to your new class.
