I've often come across this problem where two hard drives are run on the one IDE cable, both set to master.
Sometimes you get away with it.
More common scenarios are:
One or both drives have totally fried controllers, and won't detect at all.
One or both drives have partially fried Pcb controllers, and will never work again as master boot drive, unless you replace the controller Pcb. However, they will often quite reliably as a slave drive for backups.
Only other exception was with some of the seagates, where you could fool them to master, by changing them from "Master, stand alone" drive setting, to "Master, slave (or sometimes non ATA slave) present".
Bottom line, tho: I wouldn't trust a drive like this as my mission critical boot drive