How do you wire a current sensor to multiple fans?
It does not matter if you run two separate cables to the two fans from the same point - two homeruns - or if you daisy chain, with one cable from the switch to the first fan, and a second cable from the first fan to the second.  Either way, they'll be in parallel electrically and the load through the switch is the same.
However, you do have another issue.  That switch in your link is handy because it's self powered, but it is rated for up to 2.5 amps at 120VAC, so it's really not up to running even one of those 3 amp fans directly.
You could use that current switch to operate the coil of a relay / contactor and have the contacts of that additional relay switch the fans.   There are zillions of such devices available, you'd just have to select one that's rated for the application (and of course you'd have to install it correctly).
You can also find current sensing relays that will combine both functions in one device.