Problems with multi-booting openSUSE and RedHat in UEFI

Hello i am not able to multi-boot properly openSUSE with redhat in a **UEFI system.(can’t get both in the same grub menu).
Current Situation:
I have Linux mint and openSUSE tumbleweed installed in HDD. I made red hat 7.3 as a third install, after the installation finished i got the red hat grub menu only with linux mint( of course couldn’t log into mint[can’t find command ‘linux’ ‘initrd’] ). Tried fixing it via mint live cd after which i got grub menu populated with names ending with .efi extensions. opensuse
.efi **got me back the tumbleweed boot screen. So if i try getting the redhat grub menu i don’t get the opensuse tumbleweed+linux mint menu and if i get opensuse menu(along with mint) the redhat doesnt show up. So nuked the redhat install.

Now i have installed a second openSUSE tumbleweed and red hat 7.3 as dual boots inside VirtualBox UEFI mode (to test and fix them before installing redhat again in main HDD along with tw).
The same problem exists here too in the virtual machine too.
If i run ‘grub2-mkconfig -o /boot/grub2/grub.cfg’ from redhat and restart i get the tumbleweed grub menu and if run the same command from tumbleweed and reboot i get the redhat menu .
How do i fix this and get all the OS in a single grub menu ? i am really confused and looking for some guidance on this multibooting setup as i may install 1-2 more distros.
I have separate ESP-FAT partition(550mb) . Is it recommended to have separate /boot partition too for individual distro (recommended size if yes)?

PS: In my another BIOS only machine my openSUSE42.2 install disappeared from grub after installing redhat7.3, but never had problems with Linux Mint and openSUSE installed alongside.

You have to be sure to use the same boot method. Be sure you are not booting the installer in legacy mode.

yes it was installed in uefi mode and not in legacy mode, as legacy mode was disabled in settings . The /boot/efi/EFI/ also has redhat entry.

It is not “recommended” - it is required. Every distribution must have own /boot. Whether /boot is on separate partition is up to you - at least with grub2 this is not really mandatory anymore, it can be on root filesystem. But every distribution you install must have full separate and independent /boot directory.