How can adjust the grub.cfg called when rebooting?

I have installed two debians ,one is in /dev/sda1 ,other is in /dev/sdb1,and there are two grub.cfg in the pc,
one is in /dev/sda1/boot/grub/grub.cfg,other is in /dev/sdb1/boot/grub/grub.cfg.
I find that the pc always call the /dev/sdb1/boot/grub/grub.cfg to use when rebooting,
how can make it call the /dev/sda1/boot/grub/grub.cfg to use when rebooting?