Hi @paulf, welcome to SYSTM!
Currently you can not directly upload from SYSTM to ridewithgps.
But as you suggested you can dual record with your Head Unit, there are lots of people doing it for various reasons: Dual Recording—why do you do it?
Getting the data to ridewithgps would be one reason. This would be more convenient than uploading the export from SYSTM or the mail to ridewithgps.
When doing it your devices need to support multiperipheral Bluetooth or you have to use ANT+ or mix BLE with ANT+.
There may be more import/export options coming to SYSTM in the future, when the outside ride integration and/or Analytics Features are coming. We will have to wait and see.
I would hope that they are going to support the same sites Wahoo is already supporting with the ELEMNT Headunit and the iOS/Android Fitness App, since they are already invested there. Luckily ridewithgps is among those.