Fireboy and Watergirl 2: The Light Temple adds the mechanic the sequel is named for — beams of light that you redirect with mirrors to open doors and activate mechanisms. Everything else carries over: Fireboy walks through lava, Watergirl through water, green goo kills both.
The light puzzles change the co-op dynamic. In the first game one player usually waited on a switch; here both are often repositioning mirrors in sequence.
Light beams and mirrors — you redirect beams onto sensors to open doors, which adds a second layer to the co-op puzzles.
Yes — control both characters in turn. It's slower but every level is solvable solo.
Yes — two players share one keyboard, with nothing to install.