The Arduino interfaces with your PC and Simhub real time in order to output the signals that make the Gauge display the right speed or boost. I'm not sure if 1 Arduino can do multiple gauges, although I can't imagine it wont be able too.. Your best bet would be to ask this on the Simhub Discord, which can be found here https://discord.gg/nBBMuX7
I imagine it's possible although I am not sure which wiring you'd need to use for the second Gauge. You can always try to wire that to some of the other Digital pins and splitting the ground.