/diy/ - Do It Yourself

I am new to using Eagle and designing PCBs so I was wondering if anyone could tell me if this is possible. I am trying to design a board that is extremely thin (every 0.1 mm counts) that uses an esp32 module. The module is designed as a SMD component and should be soldered on a side of the main PCB, however that is not what I intend to do. I would like to design the rest of the board around the module and connect them together side by side (castellated holes to castellated holes). This would save me 0.8mm of wasted thickness on the module being on top of the main board. How could I do this in Eagle? I shaped my board to have the space missing for the module and placed vias along the edge where they line up with the modules vias but just can't seem to figure out how to get them to actually connect so the the autoroute works. Anyone know how to point me in the right direction?

Pic related in case my explanation wasn't clear. Sorry for poor paint doodle.