Drac & Franc is a co-op dungeon adventure with two monsters as the heroes. Each has different abilities, and the levels are built so that neither can finish alone — one holds a switch while the other crosses, one reaches high ledges while the other moves heavy things.
It's the co-op puzzle-platformer shape that Fireboy and Watergirl made popular, in a Halloween costume.
Yes — you switch between the two characters. It's slower than co-op but every level is solvable that way.
Same shape — a two-character co-op puzzle platformer where each character has abilities the other lacks — with a monster theme.
Yes — two players can share one keyboard. Nothing to install.