What your talking about will not make the servos work correctly. The servo pointing in the opposite direction won't make them reversed. The servo arm will. To do what your talking about you just mount one servo arm up, and the other down. I don't recommend it though. It could change geometries from one side to the other, although minute it might seem, you really need them identical.
Mixing directly in the radio will accomplish this, and allow for the minute adjustments of travel differences that may occur.
Expert brand(available at Horizon) makes a very nice servo-reversing Y-harness. I think the part# is EXRA125...it has an adjustment for travel built in that allows for the differences in servos/linkage. I've done them both ways(radio and the reversing Y) and either one works equally well. The reversing Y though does allow radios that don't have the mixing option to be used.