I have a fireblade CBr900 BEC.
I have just fitted a gear indicator display that works great and for £15 from eBay , a cheap effective gadget.

When I selected neutral it displays 0 as it should from the neutral wire but I can't get the original neutral dash light to work with it as a 2nd notice it's either have the light or display. There are 2 wires coming from the back of neutral light one brown that I'm feeding off for the neural signal and another that has 2 wires coming into one, but I have had to disconnect this to use the neutral display on the gear indicator.

Any idea how I can get both to work at the same time?

Bike neutral switches generally just give an earth connection when in neutral, so I suspect of your wires to the light one is a 12 volt feed and the other is the wire from the neutral switch which is earthed when in neutral. You can use that one for both the gear indicator and your dash light.

I don't think simply connecting the neutral light and the indicator will work, in fact you may damage the gear indicator if you try it as the wire exiting the neutral lamp going to the neutral switch will be +12v when the gear indicator wants to see an earth to show neutral.

Easiest solution would be to stick a relay in series with the indicator lamp, keeps both circuits independant of each other.