The Gibberlings 3 Tweakpack has an option to do exactly that. The latest version is the Tweaks Anthology, found at the link below, but it's technically still in beta so if you have trouble with it (I haven't) you could always use the BG2 Tweak Pack instead from the Gibberlings 3 website. Note that you use the BG2 version for all the Enhanced Edition games.
Just FYI the tweakpack doesn't fix some of the dualing options, like half-orc fighter → druid or some such. For that you should dual into cleric then keeper, then right before leveling up use keeper to remove all your level 1 spells (which will be cleric spells). They'll get replaced on level up.
Is there any line that can be added in baldur.ini to do so, as far as you know??
No, and nor should there be. Configuration files like baldur.in should NOT be making changes to game files. Either download a mod or live with what is in the base game.
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https://forums.beamdog.com/discussion/48165/testers-wanted-tweaks-is-dead-long-live-tweaks