The Mega-Cart is a single cartridge for both PAL & NTSC Vic-20 computers.  All items inside were either originally made for both video standards, or if not then they have been fixed to work with both video standards.  You need just one Mega-Cart for no matter if you own a NTSC Vic-20, a PAL Vic-20 or both!  There are just few exceptions (listed below):

PAL only (Demos in the Extra menu):
Munakois
Robotic Liberation

PAL only (Utilities):
Hypra System

NTSC only (Cartridge games):
Bandits
Capture The Flag
Final Orbit (Bumper Bash works on both PAL/NTSC machines)
Mountain King
Spider City
Tooth Invaders