In future Geocart there will be Superglue For Maps.
It will allow gluing based on one point, preserving scale and orientation of both maps, or based on two points, adjusting the scale (which will affect area distortion) and orientation of a chosen map to fit into the two points. The user picks one of these options. The points may be defined as a geographic coordinate, rightmost point, leftmost point, upmost point, downmost point, and the previous four restricted to a certain area.
Hm. I think, perhaps, that would be best achieved by creating two maps, as you have done. In the general case, you can’t glue two projections together and expect the graticule elements to match. It only works with certainty if the two projections involved are strictly pseudocylindric or cylindric.
I didn't suggest this to fail aside from exact fit of graticule. Supergluing on 1 point (preserving scale and orientation) or on 2 points (affecting scale and orientation of one map) should work fine, but there may be overlap.
The two maps are actually supposed to be there for the function to work, selecting these, similarly to blending. Leaving these two maps as-they-are is not perfect because of the need to select both before moving, need to change to distortion visualization individually, etc.
