I think that's an issue with equal masses, but mars through earth? Weird.
If you use the collision simulation thing and smash objects at high speeds, they can pass through each other.
I've noticed that occasionally, if the collision is slow enough and the orbital trajectory is right, the collider will sort of skim right on the surface of the larger object and not really merge. The diameter not changing with the mass will have to be fixed first to see what's happening there.
Also, sometimes, if the object is big/massive enough and the trajectory is angled enough, the impactor will sort of glance off rather than shatter into a million fragments. The mars colliding with earth sim is a great example of this and could actually be used as a testing ground for more realistic collisions between two large masses.
Oh yeah, supernova remnants and planetary nebulae (sunlike mass star death) somehow broke this release.