I guess so
As stated in the manual, the AD&D Players Handbook and on
http://www.planetbaldursgate.com/bg/...ciencies.shtml
a wizard would get 1 prof. slot initial and 1 slot every 6th level.
Imoen, who I dual classed to a conjurer, just reached level 6 and got a new weapon prof. slot.
I guess that goes for the other classes too. I.e. a fighter gets 4 slots at lvl 1 and 1 extra at lvl 3, 6, 9, etc.
Insane