First thing to remember when you are soldering two pieces of copper is that you MUST TIN the areas you want to bond together. (TINing is the process of putting a thin layer of solder on the copper, just as you would be soldering something, but you are just coating it. In case you didn't already know.)
THEN put them together and apply the torch until the TIN melts and solders the two pieces together.
With copper as big as that, it will take a LOT of heat.
Use an old UNPAINTED board to put the pieces on so they are insulated and the heat will build up quicker.
Hope that helps...
