Dear Miss Snark:
I, too received a rejection with a 'keep up in mind for other projects' notation written at the bottom. Now, I have other projects, but is it wise to start querying a second manuscript while other agents are looking at partials, and the number one agent on your list is reading a full?
And perhaps, the bigger question: At what point do you decide to stick a manuscript into that 'under the bed' file and move on to pitch the next?
Pats to KY, and help yourself to my Bombay Sapphire.
"Eagles may soar, but weasels don't get sucked into jet engines."
Oh, that is Miss Snark's new motto! Call the armorer for rewelding the escutcheons!
First, I think you should keep as much stuff in circulation as you can afford and keep track of. If Agent A has a full on Manuscript A there's nothing nitwittish about sending Manuscript B to Agent Q. Just keep good notes.
I'm not sure when you retire something to under the bed. There's no hard and fast rule for it but as you query you'll get a sense if one ms is working better than another. Offhand I hope you're querying me with the fourth or fifth novel and the first three are our little secret (well, ours with KY..he's big on dust bunny patrol).
The only thing that's absolutely wrong is to quit.