In theory I would agree that common ground is not as good as separate one, but between these two grounds you've got only 2.2Ohm, you think that is enough to prevent crosstalk, because I don't really think so. You know 2 Ohms is nothing. IMHO it is more about you either should have two grounds, or one that is twice as thick.
I shoveled too much snow today to want to do any math right now but my educated guess is that when it comes to crosstalk a 1 ohm resistance forming an attenuator in the network with 18 Ohms headphones is substantial. Even 0.5 Ohms seems like too much. I'll do the math another day, but my gut instinct says don't do this. My 38 Ohms HE-500 cans have separate ground wires as well, so does my HD600's (300 Ohms) and .......... There must be a reason that so many good cans keep separate ground wires.