I think your approach will work just fine--much better than switching cables in and out, which introduces a time delay that will be far more confounding than any minute increase in load from having a second set of cables attached.
As long as you're going to this trouble, get a friend to help you do the listening blind. He can switch back and forth between A and B (at your instruction, so you can listen to them for as long as you like). Then let him flip a coin to decide which is "X," and see if you can tell which it is. If you can't score 80% on a reasonable number of trials this way, you can be reasonably certain that the two cables are sonically indistinguishable.
As long as you're going to this trouble, get a friend to help you do the listening blind. He can switch back and forth between A and B (at your instruction, so you can listen to them for as long as you like). Then let him flip a coin to decide which is "X," and see if you can tell which it is. If you can't score 80% on a reasonable number of trials this way, you can be reasonably certain that the two cables are sonically indistinguishable.