New Speed Sensor + GSC10 for cadence

I don't think you can do that as the pairing options will not support pairing to both a speed sensor and a combined speed/cadence sensor.

Does the speed not work for you with the GSC-10?

I had the "old" GSC-10 sensor and now have the "new" magnetless sensors. In the menu MENU> Settings > Bike Settings > [Bike 1] > ANT+ speed/cadence have now 4 sensors listed:
- ANT+ Spd/Cad.
- ANT+ Speed
- ANT+ Cadence
- ANT+ Speed and Cadence

For the "new" magnetless sensor you have to select "ANT+ Speed and Cadence". If you select "ANT+ Cadence" that will work for the cadence but the speed will be derived from your GPS signal. If you ride in areas with GPS shadows, you may get false auto stop/start because the speed drops to zero if there's no GPS signal. If you pair with "ANT+ Speed", you will have speed but no cadence. The abbreviated "ANT+ SPD/Cad" is for the GSC-10.

If you still have the GSC10 installed, why not use it for both?

If you still have the GSC10 installed, why not use it for both?

I have GSC10's installed on 4 of my bikes, but have been less than impressed by the reliability over the past 3 or 4 years.
I find the speed sensor is the first to fail in a GSC10 and I'm not too keen on the ugly new cadence sensor attached to my crank arm by a rubber band.

It's really no different from having a crank magnet there. You get used to it quickly.

My crank magnets are tucked neatly away in the end of the pedal spindle, you hardly see them.

And so would the sensor be. You'd only see the bands. Granted, there are three bands rather than one zip tie.

You'd lose the big ugly black thing sitting on your chain stay. You'd gain a big ugly black thing spinning around on a hub (still surprises me a little to see it), but if you put it on a rear hub, you wouldn't notice it as much. But that you don't seem to mind anyway.

As for failure rates, since the new sensors haven't been out long, no one will know for sure. Mine haven't failed yet, and I've had them for a couple of weeks now. Long term? Who knows? My GSC-10's have lasted me for years, although I had one replaced under warranty a few months after I got it. Here's hoping the new sensors will last at least as long.