Awkward protocol question

Well, you can send a response to Agent B saying that you recently received some new feedback you think would be helpful and want to revise before sending to him/her. You could ask if s/he wants to see the manuscript that's ready now or wait to see a revised version.

If you do take that route, it might be wise to carry out the revisions quickly. You don't want the requesting agent to forget you.

I've been in this situation, and honestly, I couldn't make the revisions quick enough for it to be worth keeping other agents waiting. A serious revision (not a quick matter of cut-this-scene, add-this-other-one) takes time.

I'd weigh your options carefully. Even if you have to send Agent B the current manuscript, you can always stop querying until revisions are complete, so for any future full requests, the revision will be ready to go.

Personally, I would send the as-is full to Agent B. Who knows what advice she'd have? Maybe the things Agent A didn't like Agent B would.

How confident are YOU with your book? Did you agree with all of the revisions?

Agent's opinions are subjective as well. I had one agent say she wanted to delete half of my book and have me continue on from where I finished the narrative. That hurt a bit, but also made me realise we just might not be a right fit for each other. I'm not a super wham-bam actiony kind of writer, and that seemed to be what that agent preferred. Doesn't mean she was wrong, either--everyone has opinions and tastes. I found another agent who absolutely adored it as is and wouldn't dream of chopping half of it. She still had suggestions for improvement but they fitted a lot more with my vision for the book. And she's the one I went with.