OK: apologies to poets to whom all this is obvious… (And feel free to correct/amend if you know otherwise, or disagree!)
If a publication states ‘No simultaneous submissions’, it means: do not send the same poem(s) to another publication until you’ve had the ‘yay’ or ‘nay’ from us.
If it states, ‘No multiple submissions’, it means: once you’ve submitted to us, please don’t send us anything else, until you’ve heard the success or otherwise of the first submission.
Meanwhile: poems should not be double-spaced, the way you would double-space prose for submission purposes. I like to line my poems up with the ‘5’ tab on the ruler at the top of the screen in Word, to centre them, but I’m pretty sure it’s just as acceptable not to indent them – of course, if you’re playing with form and shape of the poem on the page, this won’t be relevant for you!