If both systems are on sdb, it does not matter on boot loaders as much. Both will depend on sdb as even if grub is booting from the MBR of sda, it still has to find the rest of its files on sdb.
But Boot-Repair should let you install a Windows like boot loader to sdb, or you can use a Windows repairCD which you should have to fixMBR.
How to restore the Ubuntu/XP/Vista/7 bootloader (Updated for Ubuntu 9.10 - grub2) - talsemgeest
Or from Ubuntu you can just install lilo to sdb. Lilo works like Windows in it has more boot code in the partition boot sector and chain loads to the partition with the boot flag. Lilo is better than Windows in that it will boot a logical partition. But you do not want the full lilo, just the MBR boot loader.
Restore basic windows boot loader - universe enabled if error on lilo not found
Simply open Synaptic and Settings > Repositories and tick the box against the Universe repo in the Ubuntu Software tab. Close that window and click on reload before installing lilo with Synaptic or command line.
sudo apt-get install lilo
sudo lilo -M /dev/sdb mbr
May show error messages about the rest of lilo missing, ignore, we just want MBR with bootloader.