A major issue for me would be that you'd have to write interaction for all of the BG1 NPCs that got carried across into BG2. And, uh, you know how much work BG1NPC was. (Yes, this consitutes an "acceptable transition". Anything that lets you carry them through with little to no dialogue is just a crappy hack.)
It's also really not acceptable having to start the game with the "Tutorial" button. If anything, you'd have to separate the ToB starting screen into BG1 and BG2 rather than SoA and ToB. This leaves SoA-only players out in the water, and even for those with ToB, removing the option to start in ToB isn't a very good thing to do.
Those are just a couple of thoughts on the creative/technical side respectively.