I thought I'd seen this one answered already, but I can't find it on a cruise through the archive.
If you've got an Agent SlowPoke who requested A, but hasn't responded on it when you start querying B, what do you do?
Send her a note that says, "Hey, that thing you asked for two years ago? Old news. Wanna see my latest and greatest?"
Send her a query that starts out, "You requested Old News on (date). Since then, I've finished New Hotness. Would you like to take a look?" (Ooh, personalization!)
Query her just like anybody else, preferably early in the process so if she requests New Hotness, there's a chance she'll read it before you finish Even Newer, Even Hotter?
(Author) who is, yes, almost done with New Hotness (where have I heard that before?)
Why are you querying someone who hasn't responded to your last full?
It seems to me that absent a compelling reason (ie Agent SlowPoke is ME) you'd cross that agent off your list. Or at least put her way way down the priority list.
If she hasn't responded to your nudging, that's a very very bad sign.
If you haven't nudged, well, that funny sound you hear is a wet noodle heading toward your noggin, courtesy of me. I've found things misfiled, mislaid, mislabled ONLY after an author very politely said "Hey SharkForBrains, what's up with your dawdling??"
If you really want to query her for New Hotness, just go ahead. You don't need to mention Old News.