If you use the newest sensors that work with both Bluetooth and ANT+, you can run two computers simultaneously. I have used the Wahoo sensors with success in doing just this. Run your 810 on the front handlebars and then run a smart phone or smartphone plus RFLKT+ on the rear bars and you will be able to have dual displays on your tandem.

The Speed and cadence sensor will get you both speed and cadence in both BT and ANT. The RPM will give you just cadence and will also do it in both protocols.

J.