No, it's not safe. Your ground potential will be different between the two machines and you will likely eventually burn one or the other out in some fashion.
Same AC source, same earth, if they're both from the same phase supply. This is no different to having a server with 2 PSUs, which they do.
BUT, you have to be certain that the home is properly grounded and wired. If it's not, you could definitely have different potential between two plugs. I've seen some funny stuff on older home wiring... like, ground wires that don't actually ground to anything.
Though I suppose if both PSUs are connected to the same outlet, you're probably fine.