Cadence sensor being overwritten

I have an issue with the app overwriting sensor registration, and I was hoping you could assist me. To explain a bit more, I have recently switched from a garmin H/R chest strap to a Wahoo TICKR X and Wahoo Cadence (mostly because Apple no longer supports the Garmin ANT+ dongle anymore. )

Anyway the issue is that although I can register the BT cadence sensor as a speed/cadence sensor in the app, if I am wearing the HRM strap, it seems to take priority over the cadence sensor, i.e. the same sensor ID for both HR & cadence, and I get no cadence data.

The only way I have found to get around this is to start the app, register the cadence sensor then put the HRM strap on. Then the app sees both units. Anyway I am hoping you can help with a better way to "lock" the saved sensors and prevent the HRM from blocking the cadence sensor details.

Anyway, for the details:

iOS 11.2.6
iMI 3.23.1