The transmitter can pair with one phone and one device (non phone). Devices include a dexcom receiver or a tandem pump.
Not sure what you mean by Tandem receiver, but the X2 “acts” like a device (non-phone) when paired to dexcom transmitter.
Tandem users can also use T:Connect app (and web version with more history) , which communicates to X2 pump, displaying both pump and cgm data, no direct access to transmitter/sensor.