Sure it's possible, but you'll have to properly prepare the external hard drive so that it can be used by both file systems. You can partition it into two partitions; 500 Gb NTFS and 500 Gb HFS+ and give each partition a unique name. The PC will backup to the NTFS partition and the Mac to the HFS+ partition.
Keep in mind, however, if that external hard drive goes down, you will lose backup capability for both your PC and Mac. I recommend keeping them separate and as inexpensive as large external hard drives are, that should not be a problem.