If I have two aligned PCBs with pads touching each other, will they bind if I reflow them?
In theory, you'd expect overlapping pads to solder together, if assembled with solder paste.

In practice, they probably would, but you'd be unable to inspect the joints.

The way this is normally solved in the industry is to use plated through half holes, also known as castellated pads. This image of a module intended for surface mounting is from sparkfun. There are many modules available like this.



The mother board will have conventional SMD pads. The centres of the holes on the daughterboard aligns with the centres of the pads. Solder paste on the board pads will be necessary. On reflow, it forms a fillet up the hole, which creates a solder joint that can be inspected.