How to query the next book

That's a grey area, near as I can tell. I've been through it (well, am still going through it, I suppose- one of those dual-subs is still live). What I did was to send out a good round of queries on the new book, to gague how it was doing. When it started getting a number of requests for fulls and partials, I sent out status checks to the agents still hanging on to book one.

I did the usual status check, and added that I'd since completed a second book that was garnering a fair amount of favorable response and wanted to aprise them of the situation. Then, after signing off, I pasted the hook for book two below my sig (which I indicated in the body I would do). It just felt a little less "querying multiple projects" since it was after the sig.

Two of the three agents asked for more time and requested to see book two as well. The third never responded.

I think that in part it depends on how many agents you queried for book A and how long ago you sent them query letters. You make it sound as if you haven't received any feedback from any of these agents--good or bad. If that's true, have they gone beyond their guideline deadlines for such replies? Either way, my first instinct is to SQ the agents you queried, ask them for an update on book A and tell them you have completed book B and would like to know if they would be interested in receiving your query re same. At the same time I'd probably query a new bunch of agents about book B.