Through a quirk of the editing process, I am now in the position of having two books coming out this spring from two different (traditional) publishers, both of them on music. With one coming out in March and the other in May, is it too soon to start spreading the word (even though I have yet to see final proofs on either of them)?
*Trade* publishing.
I don't think it's too soon to start talking about them, no. I'm amazed, though, that the two publishers were both ok with these two books coming out so close to one another.
Do I mention them both at once? What about when I'm pitching to music magazines and websites?
You're actually a bit late to start working on the March release if you want it covered by magazines, which often have a three-month lead time. But this is something your publisher should do.
If I'm doing a radio interview for one, can I mention the other?
Of course you can. But your publishers and publicists might not like you doing that bearing in mind they're both coming out in a very short period of time.
Is it possible that one book will cannibalize the sales of the other?
Highly likely, I'd say. I'm surprised they're being published so close together.
Will my publishers be mad when they find out?
I cannot believe you've not told them. I think they're going to be steaming mad. I know I would be.
You should have told them before you signed your contracts. The fact that you've left it so late and you've still not told them? Awful. If they put any effort into marketing and promoting these books their efforts are going to be diluted by the presence of the second book; as will the sales be.
In your place I would now tell them both as a matter of urgency. It's not too late for the publisher of the May release to delay publication, which will give both books a better chance of sales, but it's not going to be easy for them to do this.
How can I make the most of this truly once in a lifetime situation?
You can tell your publishers, apologise profusely for not telling them sooner, and hope that everything works out.