Do you want to have a different map or the same map in those two windows? If a different, there is work-around. Have two instances of Xmind open, the easier, recommended way is to install two versions of Xmind. The technically more challenging way is to run the same installation under two different user accounts (google for how to do this under Mac). (*)
If you want to have the same map open in two windows, please, remember to turn off auto-save and do manual saving only in one instance all the time. And make a back-up before every edit session.
Hmm, since Xmind is based on Eclipse that does support split windows, I wonder if you can use those commands...
*: To be precise there is a third way. MacOS assigns every application an unique ID (App ID), so you can't normally launch the same application twice. There are tweaks how to tamper with this ID, so that you launch the same application twice with different IDs. This is not recommended unless you're at least 120% sure what you are doing :-)