Endless is a great system for beginners on the Linux world, it is more intuitive than another system. Generally, novice users don’t know how to manage a partition, so I understood the proposal. But if you really want to make a double-partitioned machine with any system and endless, one way is:
- Backup all of your data of the other systems already installed on PC.
- (an easy option is make a system image, to recovery later)
- Install EndlessOS and format your HD.
- Using a Linux Live CD, open GParted (or other partition tool) and decrease the size of the endless partition (the unallocated space must be flushed at the end of the disk to reduce the risk of problems).
- Next, install other system on the free space, as you usually do. (After complete, you can restore the backup).
- If you installed Windows, download EasyBCD, add a new entry, configure to Linux, Grub2, auto detect, and save.
- If you installed Linux, just update Grub. If it don’t works, try to install Grub on the Endless partition again.
- Now you have a dual-boot dual-partitioned Endless machine.
(Note: This way is not safe, because by decreasing the space of the partition, you can damage your hard drive or damage the integrity of your Endless system. I do not blame myself for possible mistakes, just brought a way that I used. I advise you not to do this without a backup, and if you are not sure, do it on a secondary machine, never on your main pc.)