I was thinking about creating a party in multiplayer with two chars, then transferring the save to singleplayer and recruiting the other 4 npcs.

1.) Does this (mp > sp) work in BGT at all?

2.) If it does, is there any way to keep the second character in the BG 2 part?

3.) Second "if": The first character would get all the dialogue with the other 4 npcs, right?

1) You can certainly play with 2 characters you make yourself, but I'm pretty sure you wouldn't transfer it to single player. Just play it in multiplayer without having anyone else join. I think the only downside is you'll have to click 1-2 more times to start playing. Oh, and for me I found the multiplayer menu made me crash with the widescreen mod installed, if it was full screen. Alt-Enter will let you temporarily enter windowed mode, and once the game has started you can use the same combination to return to full screen if you want.

2) I believe, and this is untested, that you can. If you save your game after beating the final boss of BG1 and before moving on to BG2 (don't worry, it's quite obvious when this happens), and then once you've got into BG2 and gained control of your character you save in a different slot, you should be able to start the game in multiplayer and import the 'second' character. How and when precisely it makes sense for them to rejoin your party is, of course, up to you to determine. If the worst comes to the worst, you can always create a 'new' character with the same voice, name and picture and then use Shadowkeeper (save editor) to change their statistics to be the old one's, and you'll have effectively got the character back.

3) Yep. Of course, you can always decide that the 'second' character is giving answers instead where appropriate, or just imagine the conversations with him.

Out of curiosity, what classes are you planning to take with your two characters?

Have fun with your suicidal party :P

In all seriousness, though, you might like to consider Ajantis for your final slot, especially if you don't need anyone else using the Gauntlets of Dexterity. He's a paladin, and can be found early on, albeit slightly off the route the game suggests for you to take. Alternatively, Yeslick, a dwarf fighter/cleric who you'll come across during the main quest. Needless to say, as a dwarf his dexterity is also pretty poor for a tank, though...