The New Duo Cartridge PCB allows you to use two 8k (2764 or 27C64) EPROMS and switch between them. This means you can have your two favourite commodore 64 cartridges on one board, this is great for lots of applications apart from just having your two favourite games at hand, two test/diagnostic packages, basically anything you need to hand - plus you can switch the PCB off without having to unplug and there's a handy reset button.
These cartridge PCBs have been designed to hold two 28 pin 2764 or 27c64 (series) EPROMs. The board maps 8k of ROM at $8000-$9FFF and connects to the Commodore 64 cartridge port. Switch to select socket 1 or 2, board on/off switch and a reset button.