Hello,
I have just installed OpenSuse 13.2 alongside Xubuntu. I don’t have any other partition.
When I reboot (in Legacy Mode), I activate Grub, but it proposes only to boot into (X)Ubuntu, not Opensuse.
I also tried to boot in UEFI mode but it doesn’t see anything to boot into.
Below I join a link to a screenshot of the Opensuse partition (sdb) in Gparted (from xubuntu of course).
Tell me if you need more details I’ll do my best to provide them.
It’s a bit hard to guess without more info. But I’ll try guessing anyway.
My guess is that your xubuntu has installed grub in the MBR, while opensuse installed grub on a partition (perhaps the root partition).
In that case your primary booting will be with the xubuntu grub in the MBR. You need to tell it about opensuse.
Boot your xubuntu system. Then, as root:
grub-mkconfig -o /boot/grub/grub.cfg
(I think that’s the right command for xubuntu – on opensuse, it is grub2-mkconfig).
That should add an opensuse entry to your xubuntu boot menu.
It works perfectly. Thank you so much ! 
For general info never ever mix boot modes in different OS. If MBR all need to be MBR. If EFI all need to be EFI. In theory it can be done but it is very painful and the different OS’s can get confused.