Are both of the routes already in CT? 

Unless the routes are very long, the easiest is to open one route and add the other as a "trace". Then, extend the first route along the "trace". 

The "Add Trace" option is available on the "hamburger" menu on the top right of the map page (where the map options are). Tap the orange button "trace GPX". You will need to have downloaded the gpx file in advance. 

If the routes are not in CT just be aware that CT may not import them as an exact copy. In which case it might be better to start with a blank map and trace the external routes one at a time. 

Otherwise as the post you found suggests, you'll need to merge them manually. 

Good luck!