Garmin Edge Explorer

Not sure what you're asking exactly. If your question is "can I join two routes in Garmin Explore app?", then the answer is no as the Explore app does not support this functionality. But there are plenty of online tools (like https://www.therideatlas.com/tools/gpx-editor/merge) that you can use two combine to GPX files with tracks or routes, and then import the combined GPX into Explore app.

If you're asking whether the Edge Explore cycling computer can combine two courses, then I don't know for sure but my guess would be that it isn't possible directly on the device.