Hmm. To my knowledge, the big publishers accept multiple submissions from a single author, unless they say otherwise. The smaller publishers (well, for short fiction, they're all smaller publishers now, sigh) might make a request that you not send them something.

Anyway. As a matter of politeness I wouldn't do multiple submissions to a single market anyway, and (unless they say otherwise) I'd wait a day before sending off a new submission (same day if it was snail mail). That's only me, though: I'm sure there are others who feel differently.