Updating regulator to support 2 kegs?

I have a single regulator and my brewing has finally caught up that I have a 2nd keg. In hindsight I wish I had bought a double regulator so I could fast carb new keg at high pressure while keeping other at serving pressure.

Anyway, I decided to upgrade my current regulator with a Y connection so I could separately shut off gas to each line. Problem: I can't remove the existing value to replace with Y valve. I have been using wrenches with a lot of pressure but it won't budge and I'm worried about breaking the regulator.

I think I've given up with this approach but just wondered if anyone has made this swap before.

Picture of regulator and Y-valve