EasyBCD does indeed work but you need a stable booting OS to install it upon to add the other HD's OS.
However your BIOS should save and boot the choice of HD's, so I would reset BIOS to defaults in case a bad setting is interfering: Clear CMOS - 3 Ways to Clear the CMOS - Reset BIOS
Then boot into BIOS setup to set preferred OS HD to boot first, followed by secondary HD. Make sure you Save the changes using the key given and Exit Saving Changes.
Confirm you've done this.