1 Campaign, 2 Parties, 3 SAME PLAYERS.

[Otaku]

If you really want to have two separate groups, that should be fine with sufficient record keeping and/or mechanically connecting any PCs where it would be appropriate. B2 is gonna be a new character, so if they want to come up with something that connects them to B1. Like the classic mental link (be it psychic, magic, spiritual, etc.) between two twins. Otherwise, yeah, players need to be able to compartmentalize and not take advantage of what they know that their character's don't.

However, my old high school gaming group (back under Third Edition, Revised) had session where almost everyone had invited a new player. They were totally new, so right or wrong I think most of us also built a character for them to pilot, at least for the first session...

...none of them showed up. The GM had planned an adventure for a larger group with the new characters, so he asked if we wanted to run a second character. We did, and it was fun. Yeah, we had to be cautious not to unduly favor our pairs of characters or act upon information we didn't have... which was easier than it sounds. Instead of separating our two characters, we just often paired and either roleplayed them already having a (mundane) connection or developing one quickly.

Anyone who already had an handle on GURPS, would run two characters. When newer players joined the group who were learning GURPS, we only had them start with one character, and the option to bring in another later. We had more than one GM in our group, running different campaigns, and we ended up running multiple characters in a WEG Star Wars campaign, as well as another GURPS (Third Edition, Revised) game.

Yes, it can and will create some problems, but at least as a player, I found it to be worthwhile.