I'm thinking that the barycenter could be geostationary to a surface point on both planets... as in linearly. That way we wouldn't need to account for N-body physics, the game could view it as two balls connected by a stick, which for us would be invisible. Also, theres no need to think of it in normal orbit/rotation terms.. if they were locked into relative geostationary orbit, the days would be different due to eclipse by the other planet, and the whole thing would be a spinning ball and stick molecule (I'm too lazy to look attach a picture, but think the ball and stick molecule of CO2). Edit: To make this essentially a 2-body problem than a 3-body problem, incredibly decreasing the complexity, the planets should lie in a distant orbit from the sun, so that the gravitational field does not affect each planet differently on its closest and furthest approaches to the sun. I suppose, however, orbit would be a 3-body calculation... So yeah a distant di-geostationary orbit could be doable I think.