- create a board "B1"

- on B1 add a schematic, add some components, wire them together, convert to PCB, route the traces

- right-click B1's schmatic -> "Generate/Update block symbol"

- create a board "B2", open B2's schematic

- right-click B1's schematic -> "Place Block Symbol", place 2x B1's symbol on B2's schematic

- convert B2's schematic to PCB.

NOTE: it just adds the components from B1's blocks to B2's PCB, it doesn't retain the layout OR the traces.