I cannot get split screen to work within Mojave at all. I have not tried with an external monitor as my macbook pro is primarily mobile and my only apple machine. Just wanted to throw my two cents in. It would be really nice to have this feature as it works in Windows without issue. I find it frustrating that the wide majority of applications in Windows can utilize split windows with a resizable frame when Mac OS X can only support a limited range, VMWare being a glaring omission.
Or I'm totally wrong and any of you are free to shut me up. I am a novice mac user.