that answers the opposite question.
The question was: Endless OS is installed, can I install another Linux in parallel?
I was able to boot an usb-stick with a heavily customized Ubuntu 16.04, but this answers yet another question 
As far as I understand Endless OS, the modified Grub 2 creates its grub.cfg on the fly from available kernels. I don’t know if this still works if another Linux installs its Grub 2 and where the corresponding module(s) could be found.