Well like you said, most of your current party is made up of inseparable duos, and it is probably not in your best interest to lose two party members for one. However, there are a few ways to ditch one NPC without his/her counter-part leaving as well. First, you can lock the undesired NPC in the closet, so to speak, by selecting that NPC and having them enter a secluded building or area that you will not use again. Then reform the party dropping that NPC from your party. He/she will not be around to talk to you about your decision to drop them, so then their counter-part will have no reaction either. Be sure you never enter that particular building or area again though, since they will then confront you and their companion will get angry and leave. But as long as you leave the NPC stranded his buddy will never know the difference.
A second method is simply to stage a convenient "accident" where the undesired NPC ends up dead. This can easily be accomplished by stripping the NPC of all his/her equipment and placing them at the front line in the next battle. Yes it's tragic and their counter-part will mourn the NPC's death... but they'll get over it
Now you are free to add a new member to your party! Brawen, the cleric you find at the carnival, is a pretty good cleric in my opinion. Jaheira is alright, but I never liked her snooty attitude.
--Darkman