I figured out this little trick using the new Smart Actions feature.
I won't include a code, I'll just tell you how to set it up so you can apply it to any game.
Step 1: Open the Nexus Manager, load your game of choice and go to the Bindings tab.
Step 2: Add a new binding using your preferred "shift" button. I use Left Stick Press.
Step 3: Open the Smart Actions editor for this binding. Set it to execute On Press. Now add the following actions:
- Enable Binding Group 2
- Disable Binding Group 1
Step 4: Repeat steps 2 and 3, but this time set it to execute On Release. Now add the following actions:
- Enable Binding Group 1
- Disable Binding Group 2
Now you should see the following:

Step 5: Add the back button assignments you want to use when the "shift" button is
not pressed. In this case we'll use ABXY. Add these to Binding Group 1.
Step 6: Add the back button assignments you want to use when the "shift" button is
pressed. In this case we'll use the DPAD buttons. Add these to Binding Group 2.
You should see the following:

And you're done! Now there is no need to ever take your thumbs off the sticks. The back buttons are bound to ABXY, and when you are holding LS down, they are bound to DPAD instead.
You can verify this works by pressing LS (or whatever shift button you used) in the Manager. You should see the Group 1 bindings become disabled and the Group 2 bindings become active, and when you release the button they will go back to normal.

Enjoy