OK, understood. Shame.
You can upload a a route that is not on plotaroute and then combine it with one that is already on plotaroute, but you can't combine two routes that aren't on ploraroute I'm afraid without uploading and saving one of them first.
The existing method seems to be limited to joining routes that exist within Plotaroute.
Is it possible to upload a route, then directly append another uploaded file?