There’s no way to do that on cycle.travel, though you can plan several routes in one folder, and then see them all together on one map (from your ‘Journeys’ page). But I think you can use the gpsbabel program to merge several GPXs into one file.

Very easy to do if you know your way in a text editor and understand what xml syntax is. Just copy and paste then drop one file to the other.

Or use VisuGPX and EditGPX online.

Thank for information

I also like RouteConverter for tasks like this.

https://www.routeconverter.de/home/en