Hi CoaXing,
The way i run multi-disk games is as follows:
Hit Start and select "attach d64/t64/dir..."
Select the first disk, press the A button and then choose "load first program" from the menu.
The game will load as usual, but when it asks for the second disk do exactly the same as above, but when you go to "attach d64/t64/dir..." choose the second disk instead. Remember to select "Load First Program" after you have done it, and the game should automatically detect the insertion of the second disk.
As for the previous two people that responded, either help the guy or shut your traps. It's really quite easy.