Copying these 2DA files to override will do nothing, unfortunately (I've tried it before). That is because those 2DA files compiled to baldur.exe cannot be overriden.
I've asked dev in regards to removing hard-coded restrictions in regards to those files since I really want Monk/Sorcerer multiclass. Hasn't been implemented yet.
Racial restrictions can be overridden in 1.2; it was one of the externalized features. I've done it myself. I don't remember if I ever posted about it, I'll look into it.
Nowadays you can pretty much alter whatever by popping up NearInfinity, extracting the corresponding 2DA and slapping your edited version in the override.
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I've asked dev in regards to removing hard-coded restrictions in regards to those files since I really want Monk/Sorcerer multiclass. Hasn't been implemented yet.
http://www.gibberlings3.net/bg2tweaks/
I've done it myself. I don't remember if I ever posted about it, I'll look into it.
EDIT: Here it is:
http://forum.baldursgate.com/discussion/comment/426451/#Comment_426451
Nowadays you can pretty much alter whatever by popping up NearInfinity, extracting the corresponding 2DA and slapping your edited version in the override.