You can use a "Y" harness, use a pmix, or a reversing "Y" harness. Lots of options. With a 6EX, if you use a mix, the second elevator channel will not respond to the trim of the elevator channel, so your best bet is the reversing "Y" harness. That's because proper operation of 2 servos will require them to move in opposite directions from each other to give you the proper amount of elevator travel. Here's the part and its MSRP:

FUTM4150 SR10 DUAL SERVO REVERSER US$49.99

These parts can be ordered through any hobby retailer who sells Futaba systems and accessories.