The short answer is “It depends…”. Copper is a good conductor of
heat, second only to silver. If your flame is hot and fierce enough
then you can possibly get away with heating just the joint area, but,
depending on how quickly the heat is conducted away, you could well
find that you simply can’t get the joint hot enough to melt the
solder without heating the entire object.
You don’t mention the shape of the components; are they short and
thick, or long and spindly? Short thick ones will conduct the heat
away much faster than long thin ones. Conserving the heat with close
fitting fire bricks is one technique that can help, as is using two
torches: one large bushy flame to create a background heat, and a
small concentrated one to do the actual soldering. As I said at the
beginning, “It depends…”.
Regards, Gary Wooding