Editors who accept simultaneous submissions often say submitters should “let us know up front and inform us promptly if work is accepted elsewhere.”
Fair enough. But suppose you submit to them first and only decide after the fact to submit the same piece simultaneously elsewhere?
Do the first editors really want you to let them know that you just did that?
Every piece should always be simultaneously submitted, or you should assume it soon will be. So just put that in your cover letter anyway.