[chris_dcosta]

"On a lighter note - you could unlock them both, then see who gets in first and locks the other out, then you'll have a clear winner!"

I think there is something to be said about this approach even if it was said in jest. You might also want to change your own terms and conditions before you do this, to allow you to do this.

I doubt very much if the parties care about you at this point, they are mainly focussed on each other, and if you hand over "the keys" in an email to both at the same time, and resign your role as supplier, with the support of your T&Cs your should be OK.