It probably has to do with routing space. I just had some Sims stargaze the other night and they laid down in a big open area out on the front sidewalk. Make sure there's enough open space to have room all around both Sims when they're lying down. Any object in the vicinity could cause them to route away from each other.
If you do have the space then it might just be a routing fluke, especially if they were coming from different distances. I think sometimes when one Sim takes much longer than the other to get to the meeting spot it can trip up how the animation plays out.