Very well known and there is a lot to be found about it on the internet.
You can play with two players at the same time or alternately ... exchange.
If you started with 1 player, but you want to continue with 2 players?
Press F1 and then F5
With Nemesis 2 in slot 2 you can really play the last level.
This disk image will ask if you want the Nemesis 2 option.
Of course again links to sites with even more tips.