Hi John Thomas,
I just looked these up on the ANT+ Product Directory (
http://www.thisisant.com/directory/) and it looks like the VDO Z2 supports separate bike speed sensors and cadence sensors only, and the bontrager duotrap digital speed/cadence sensor only transmits combined speed and cadence. You can see this by the icons associated with each product in the directory (SPD or Cad vs S&C).
We strongly recommend displays to support both kinds of speed and cadence sensors but it isn't always the case.
If you also use a sports watch and or phone while you ride you may be able to pair the speed and cadence sensor to those.
Kat
