i really cant see why they dont both wander if they both have the ability ill look into it,
also maddawgs comment of that lots of units wandering could cause lag all depends really, if they somehow all wander at the exact same time it will most likely cause a spike but if they just go about it randomly it shouldnt be to bad as its just movement and doesnt involve triggers or any advanced game commands.
Tested it out i went over everything and i cant find why they wouldnt wander cause all mine did, i gave 20 units wander all had different specs and they all wandered, make sure its an ordinary ability though and not a default active cause a few had that and they didnt wander