Simultaneous Submissions

No, it's best to let each ms./ project idea stand on its own, PB or not. As for PBs, sometimes agents want to see the completed ms., since they're usually under 1k. BUT as for that, check each agent's guidelines.

There are a couple of agents who specifically ask you to query more than one project at a time. All are children's agents. See their guidelines. Erring on the side of caution says not to do so unless it's specifically asked for.

This is a useful thread, thank you!

I submitted two MSs in one go to an agent whose guidelines said minimum of one MS, maximum of 3 MSs for PBs. I was wondering what to do when I submit to the next agent I'm looking at that doesn't say to submit more than one...and now I know the answer

Like the others said, this might be considered a big no-no. You should definitely query them separately unless told otherwise. Or better yet, wait until you get representation from an agent and then pitch the second project to that agent instead, but be sure to query only agents that represent both of the genres you write.