Most of the people I've been going to school on to learn about the publishing industry are editors, and editors will tell you that there is no sin graver than that of simultaneous submissions. Simultaneous submissions merit blacklisting for life--as far as I can tell, editors are unanimous about that. You can query the whole world at once, but the minute somebody has your full manuscript, your job is to start writing some other book, because you are about to spend a year with your foot nailed to the floor while you wait to hear back from the one person who has the ms you just submitted for consideration. It's annoying, but that's what there is.
That's what there is for an unagented writer dealing with editors. Did I mention that I overgeneralize?
It turns out that agents don't assume they have an exclusive claim on consideration of your ms unless they've asked for it explicitly. Even if they've asked for and received the full ms. And if an agent does ask for an exclusive, the writer is expected to grant it for a short period of time, usually a few weeks.
Why did no one tell me? I think it was because A) I didn't know I didn't know, and so didn't ask, and B) I had just enough other clues that I was starting to look like I might know what I was doing. Oops.
Since October, the big book has been somewhere in the process over at the Shiny Young Agent's Shiny Young Agency. I have been laboring under the erroneous assumption that, because I'd sent her the full ms, that meant I couldn't send it anywhere else until I heard back from her. Simultaneous submissions are a hanging offense, after all, if you ask an editor. I don't know what the Shiny Young Agent's assumptions are, but I gather now that agents assume entirely different things. She would be within her rights to assume that I've been shipping the big book around to everyone under the sun. She didn't ask for an exclusive, so she's under no obligation to read it in a timely way. It's nobody's fault but mine that I didn't already know this.
As annoyed as I am at my own ignorance, it's good to have a new plan. Once the current short story is fit to mail out to magazines, I go back to the big book to fix the second half of the ms. No more waiting for feedback that will come whenever it comes. I can probably fix the big defects in two months. It's not likely that anyone really wants to see a 300,000 word first novel, but then, it wasn't likely that the Shiny Young Agent would ask to see it, and that happened. If that unlikely event repeats itself in June, I don't want to have to say--again--that the ms will need several months of revision before I can send it out.
There are now two agents who have said to me, quite explicitly, that they do not rule projects out on the basis of length alone. Even if it's 300K? No, not even then. When I have something query-worthy, there are people to query.