Since they're installed, the Y harness is the way to go.

I helped a buddy with his F-22 last fall, and we worked out the programming in his DX7 for individual hookups. It was a chore. Wound up in the Radio Forums finding out what options had to be turned OFF as well as which had to be turned on. We got it working about the time his reversing Y harness came in.

The mfg could have quite easily had you install the servos so only a regular Y would have been needed, but didn't bother. Or didn't understand the arrangement. ARFs can be quite annoying.