I'd contact the first pub and ask if they've made a decision yet to run the piece. You don't have to tell them why; just act like you're following up on the submission, which is not uncommon.
Their answer should tell you whether or not to go with the second pub. If it doesn't, then you need to tell them that you've received an offer for the shorter version but would prefer to have the essay published through them.
In my view, the best way to get out of this bind is if the first pub rejects the piece. Then you can simply go with the alternative, and no one will be the wiser. But if they accept the piece, you'll have to contact the second pub and request that the shorter version be removed from consideration. You'll find that to be awkward as well, since they've already made an offer.
As a rule of thumb, you can avoid this problem in the future by submitting your work to only one publication at a time. But I'm sure you've already thought of that by now.
Whatever happens, I hope things work out in your favor.
Good luck.