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.
Care to share whatever you found, cause I'm not seeing it. Only thing that even comes halfway close is the Runes of Witnessing/Shadow of the Warp answer. Course it deals with additional dice being used in a roll, not in a reroll............ And (IIRC) doesn't even apply anymore since SotW has changed?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.
And hence, because there is absolutely no reason to reroll and it opens up the possibility of cheating/abuse we should not bother with the reroll.