Thanks!
Spent a chuck of time working on this today after the first half of the week was mostly taken up with other commitments. Still aiming for a game at the end of the week.
I managed to get a lot of the basics in today: the two players can be moved around the map, kill monsters, progress to the next level, die to monsters, "win" the game, and so on.
I'll aim to get a gif for tomorrow but basically a turn involves issuing a direction for each player, which they will perform simultaneously. Attacks and special abilities will all be triggered based on direction.
The next step is adding in a bunch of monsters and abilities that will take advantage of this mechanic. For example: a monster that must be attacked from two directions simultaneously to be killed. Environment features/hazards that require being in two places at once? I'm not really sure just yet how a lot of it is going to work out.