I write children's stories, but I follow you anyway because your advice usually spans the genres. An agent recently requested a manuscript based on my query. It has been about 6 weeks now and I haven't heard back from the agent. No problem, I'll take that as a no, but I am attending a conference in a month where the agent will be present. In fact, I have a 10-minute query critique session with that agent. I have no intention of bringing up the manuscript I already sent, but in the slight chance that the agent recognizes my name and asks why, what the heck do I say? How do I even approach this without making it an awkward moment for the agent?
If you read this blog you've missed the part about how long it takes agents to respond to full manuscripts. Hint: it's a whole helluva lot longer than six weeks.
Right now (October) I'm reading and replying to fulls sent to me in April. That's not six weeks. That's six MONTHS. And I'm nowhere near as behind as some of my colleagues.
So, first thing: remember your manuscript has NOT been rejected.
Second: if the agent asks about it, you say "I sent a requested full on DATE." And that's ALL you say. Don't fall in to the trap of "I know you're busy" etc. We're ALL busy.
And why you're not going to ask for info about this requested manuscript puzzles me. You've had a request for a full. You have a chance to find out, in person, what the agent thinks. And you're not going to take it? Are you SURE you read this blog?
If you elect to talk about the second manuscript, and the agent says something akin to "hey your name sounds familiar" you simply say "I sent you a manuscript in September called Cool Your Jets. This is a different book."