Installing PureOS along side of Elementary OS

Ages ago I installed Pure OS and then installed Elementary OS, and it wiped my grub and boot partition. Now I broke my kernel trying to install Virtualbox additions and I only had a 4.15 kernel to boot into. So fresh install it is.

I’ve reinstalled Elementary OS, how do I install Pure OS so both can happily work together?

Del XPS 13 (9333) in legacy boot. I couldn’t get efi working.

My partitions are;
sda - gpt partition table.
sda1 - grub2.core.img, 3MB.
sda2 - fat16, 144MB, unused efi partition.
sda3 - luks exe4, 50GB, elementary OS root (file swap).
sda4 - luks exe4, 50GB, going to be for pure OS root.
sda5 - exe4, 500MB, elementary OS boot.
sda7 - exe4, 500MB, going to be for pure OS boot.
sda6 - luks exe4, 145GB, shared partition for files.

I used to have one boot partition, sda5 - exe4 1GB. Can you have a shared boot partition, or are two separate partitions best?

If the grub partition is too small, I can expand it into the efi partition.