There is essentially nothing to do. It could use some sandbox traits (spawing things, altering the state they are in) and some optional objectives would be nice. Also, it's just... Small.
Reviews for "3D 2 Player Sandbox Game"
Three main problems
1) This isn't a sandbox. Don't kid yourself and don't tag it as one.
2) What is in this game is bare minimum. Like, Atari experimentations from 3-4 decades ago bare.
3) Dull, boring, and nothing to it. You say it's a physics simulator, but it doesn't even feel like one.
Final conclusion: This will entertain that one kid from 1097 who stepped through a time portal for about five minutes before he gets bored and learns how to google porn. Other than that, there is nothing to this.