@mikefarley, this is the explanation from Tandem.
Can I use a separate Dexcom receiver to view my data, in addition to the pump?
No. A single transmitter cannot be connected to both your insulin pump and a Dexcom receiver at the same time. Dexcom CGM transmitters can only be paired with one medical device (either a Dexcom receiver or t:slim X2™ insulin pump) and one consumer device (phone or tablet) at the same time.
Can I still use the Dexcom Share feature with my t:slim X2 pump?
Yes. The Dexcom transmitter can pair with one medical device (either a t:slim X2 or a Dexcom receiver), and one consumer device (phone or tablet) at the same time, allowing you to continue using the Dexcom Share feature as you were before.
There may be a work-around for this available.