[SixOfSpades]

From Chapter 5 of Volothamp's Comeuppance:

When assigning weapon proficiencies for your first class, plan ahead and be aware of where you're going to put them in your second class, as well. Let's say you really like 2-Handed Swords, so your Fighter puts lots of points into Large Sword. Then you Dual to Cleric, and eventually get your Fighter half back—and all your points in Large Sword are gone. Well, duh, Clerics can't use bladed or piercing weapons, ever, so those points got erased. (Clerics and Druids are the only classes where you have to worry about this; A Fighter->Mage can use Halberds and Composite Long Bows if you want.) Angrily, you erase that character & start over. This time, your Fighter puts 3 points in Mace, then Duals to Cleric and puts another point in Mace, hoping to have a total of 4 when the Fighter reactivates. That won't happen, because the game doesn't add the proficency points, it simply compares them and grants you the larger of the two, so you've still only got 3 points in Mace. You've only wasted 1 proficiency star, so it's not that bad, but you'd have been better off putting it in Sling or something. This business with proficiency points is the only case in which you can add to the skills of your first class after you Dual. Fighters can achieve Grandmastery (5 points in the same weapon), and so can Dualed Fighters. Even if you only put 2 Levels into your Fighter half before Dualing to Druid, you can still put multiple points into a weapon (after the Fighter class has been regained. You can't do this with the Fighter class inactive, because Druids may only put 1 star into a weapon). Note that BG1 does not allow you to "stack" like this (put weapon stars earned as a Druid on top of weapon stars earned as a Fighter), but BG2 does allow it.