As a postdoc, how to harmonize increasing independence with relation with your supervisor
As I and others have said many times in this forum, the appropriate method to resolve such problems is communication.

It doesn't help anyone if everyone just interprets each others words and behavior without any factual knowledge what they really mean. Write B an email with content such as

Dear B,
I've given the issue of how we try to hire our graduate students for
project X some thought, and I think we're pulling in slightly different
directions. I think it would be good if we could exchange a few
ideas to make sure we're on the same page with this.

Would you be available for lunch on Thursday to talk this over?


Point being that you make implicit assumptions on why B behaves the way he does. Your assumptions may make sense, or they may not -- I suspect the latter, given that you call his actions "erratic", "feeling threatened", "wants to undermine" you. This is not how most people behave. Most people have very rational reasons to behave the way they do, but you may simply not know what these reasons are, and you will not find out if you don't talk it over in a frank conversation between colleagues. 

I would suspect that ultimately both of you have the same goal, namely making this research collaboration a success, because that is in the interest of both of your careers. On the other hand, "putting someone in their place" is not in either of your interest (in particular, not in the interest of B either if it jeopardizes the collaboration). That would imply that you ascribe B's actions to the wrong motivations, and it is on both of you to clear things up.