If you heat the partial before you quickly slide the well chilled full can into it, would that make the assembly easier?
Yes it probably would, however the walls of a drinks can are so thin that you will easily push one can into the other provided you drill the pressure vent hole first.
Hi Bonnie, When I have finished using the stove, I wait until it is completely cool, then I tip the remaining fuel back into the fuel bottle. The stove drains readily through it's burner holes and any residue left in the stove evaporates.
A good blow usually put's it out, or snuffing as hoytedow suggests.