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.
Important note about Dexcom support:
For "no restarts" alert or physical problems with your sensor or transmitter, such as bleeding, bruising, skin reaction, adhesive issues, applicator issues, or broken wires, you should contact Dexcom using one of the options at dexcom.com/contact.
If you have paired a Dexcom CGM with your Tandem insulin pump, and your question isn't answered in this help section, you should contact Tandem customer technical support for all other issues by calling (877) 801-6901.