So I think because the valve index doesn't have an input map for "Trigger Button" only "Trigger Pressed" so when you set "Generic XR Trigger button" the index controllers won't read it. So I've just added "Valve Index Trigger Pressed" to the input bindings. It's uploading now the build should be named "Welcome to the Simulation - Valve Index Fix". No rush of course to check it but please let me know if it fixes the problem, and we can also let anyone else know who may be experiencing the same problems.
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Oh that makes sense- if you use the Button action type then there's no convenient discretisation of the triggers. The action type has to be set to Value + Analog. The triggerPressed binding is for the clicky button at the end of the trigger stroke.
I tried the fix and it's playable on index now, although both guns fire when the right trigger is pressed.