As above: the re-rolls cancel each other out. The first roll is the only one that counts in such a situation.
This is not supported by GW's own line, which has been included in at least the Eldar FAQ. Conflicting re-rolls cancel each other out.While logically, they might cancel each other out, the rules would be to roll them, pick up all the dice, and re-roll everything. Silliness, but whatever.