startxfce4 command is just a wrapper around
enter-chroot, which has a bunch of options that let you do atypical things like you're asking for. Say
sudo enter-chroot --help for a list.
The one you want here is
-n, which lets you name a specific chroot. The full command would be:
$ sudo enter-chroot -n kali exec startxfce4
You could wrap that up in a shell script, but since ChromeOS's command shell remembers history, I don't bother. I just say Ctrl-Alt-T to start the shell, then Ctrl-R to search back through the history for the big ugly command I want.
I use the ChromeOS shell — as opposed to the Ubuntu shell via Crouton — for little other than Crouton commands, so there's no danger that my carefully-composed long commands will ever fall out of the history.
Incidentally, two XFCE-based chroots are going to take a large chunk of the free space available on a typical Chromebook. You might want to store the chroots on an SD card instead of the internal storage, since most Chromebooks have a built-in SD card reader. Then you need to add something like
-c /media/removable/Crouton/chroots to your
enter-chroot command. (The
Crouton part of that path is the name I've used for my SD card.)