Yes, you can use your Picture Keeper on multiple computers, both Apple/Mac and PC/Windows. It will save the backups into separate folders on the Picture Keeper. After backing up the second computer, in the View Pictures window, you will see both computers listed.
With that being said, backing up pictures from multiple computers onto one Picture Keeper can fill it up more quickly. You could use separate Picture Keepers for each computer, to make sure you always have enough storage for each computer.
If you need any additional assistance, please contact Picture Keeper Support.