I’m an editor of a small press lit mag. When pieces I am considering are accepted elsewhere, the author usually tells me, such and such has accepted such and such for publication, so I am withdrawing it. I don’t at all mind knowing the title of the publication, and there’s certainly nothing unethical about sharing it. It’s not necessary, but it’s not a problem including it either. If you have their “word” it’s being published, I’d go ahead and withdraw it elsewhere, although these things do sometimes fall through prior to publication, and it’s a shame if that happens and you’ve withdrawn it elsewhere. As a writer, I’d be tempted to wait for the contract. As an editor, I know it’s annoying to be a few days away from issuing an acceptance only to find the work is no longer available. Most editors expect you to withdraw as soon as you’ve got a verbal yes.