It is possible to merge one or more areas with OSM tools but what would it take to merge two already imported and prepared areas "on the fly"? Would be interesting for the mobile scenario (Android/iOS) where the users only want to download one area by one and routing should work across those areas. As a first step try without CH. Still the node and edge ID mapping could be complex or a graph copying is required using lots of resources. The location index needs to be dynamic instead of 'read-only'. Or the merged index needs to be created on-device.

A workaround could be to offer on-demand extraction of the bigger or combined area from a world wide graph. Still the user would download an area which is already existent on his device.

One further solution could be to implement a graph wrapper which contains multiple areas and manages the edge and node ID-offsets. Still this would not work for CH and this would require a border nodes matching.