Setting two outgroups in Treemix

Question

[devenvyas]

I am new to Treemix, and I am trying to get sensible trees. I wanted to avoid picking a specific modern population as root, so I tried including Neanderthal (n=2) and Denisovan (n=1) populations, so the tree would figure out that these populations are heavily divergent and automatically set the root to be between them and everyone else.

This appears to not be the case and they formed a heavily diverged clade in the middle of the tree. I know how to root a single population, (e.g, -root Mbuti), but how do I root a clade of two populations?