Possible to equal floating grounds?

I am making a wireless fencing system, as you can see in the schematic I included. The guard is HIGH when nothing happens. If you then push the tip of the epee down, you close the circuit (via a switch) and it gets pulled LOW by the pull-down. The idea is that if you depress the button on one side on the guard of the other, you can see the point is invalid, because the guard of the other gets pulled LOW.

The problem i face now is that each sword has its own arduino. So the reference ground of one isn't the same as the other. I.e. they are floating relative to each other. Is there a way i can 'sync' these ground. Or another solution to detect if one hits the other ones guard.

It is not possible to connect the grounds, since the system is wireless

Thanks for reading.

Ground defines voltage values. The only way to "sync" the values output by two independent sensors is to have a common ground.

I should think CrossRoads will have a good answer for this. (Google Crossroads Fencing.) Send him a PM to direct him to this thread. I'm interested in the answer too.

Check out Hitmate wireless epee fencing

I believe it uses the capacitance of the bodycord and opponent's body to ground the bellguard enough to be sensed as a low, closing the loop thru your weapon's scoring unit.

We have a couple of sets, works well with larger kids, goes thru 9V batteries quick.

I suspected it would be a capacitance-based method. Think one of those MPR121 capacitive touch modules would work for that?

This kind of thing?