Actually, just for kicks I'm trying the following in my current game:
My berzerker got 5 proficiencies in Flail and 1 in two handed weapons. Dualled at level 13 to mage. Took one in quarterstaff at dualling. Did my level-ups as normal until level 5, but I haven't clicked on "Level Up" since I hit 40,000xp (I'm at about 1.2 mill now, berzerker levels will reactivate at 1.5 million). Why gimp myself as a 5th level mage for the entire time it takes to get over a million experience points? Because when my skills finally -do- reactivate, I'll have -two- proficiency slots (one from level 6 and one from 12 as a mage) that I can put into any proficiency usable as a fighter. I plan on putting both in quarterstaff so I'll have 3 proficiencies with the staff when I reactivate (and the 19-20 critical hit from the two-handed proficiency I got as a fighter). My points at 18 and 24 will go toward quarterstaff as well, so the final result will be 5 in flail and 5 in quarterstaff with 1 in two-handed weapons.
Of course, if you like the idea but you're not quite as masochist as I am, you can level up as normal until 11 and just go through levels 12 and 13 without levelling up, and thus at least save -that- proficiency point. But I really wanted to eventually get max proficiencies in two weapons and the improved criticals for one of 'em
Qwinn