I have a manuscript (full) out with an agent who is due to get back to me soon. Last week my dream agent, to my immense surprise, requested a partial. I’m ecstatic to say the least, but also realistic. Naturally, I sent the partial right away.
According to the letter, it will take a month for my dream agent to get back to me. It wasn’t an exclusive, so I didn’t feel the need to reveal that another agent (a junior agent) has the full. Should I have? Or should I just wait and see what happens with the junior agent, because she might end up passing on it anyway?
While some people do mention in their letters that other people are looking at their work, it's no longer obligatory unless you granted an exclusive, which you didn't. In the old days it was the appropriate thing to do, but today we're just assuming that everyone is mass querying and that we are competing with someone for a manuscript if it's really good. Telling the agent (the second one) that someone else is looking at it is polite, but not necessary. In other words, you don't have to email her and go out of your way to say it (the fewer emails you bother an agent with, the better), but you can mention it in passing in some other correspondence if you have a chance. It may spur them on, or it may do nothing. Either way, not a big deal, no reason to split hairs.
Other agents might not feel this way, but certainly it doesn't really make a difference at my office when we get a letter with something like "other agents have expressed interest."
Don't wait on one agent over another unless you're in the very final stage of deciding between agents and you just need more time to think. Publishing is slow enough as it is. There's no reason here to make it any slower.