You'll have to deal with this problem on a different thought process than trying to find a compatible middle-ground compromised disk format.
You'll need to get hold of an emulator for each system that will let You handle the drives as a removable device. The Mac will run an emulator that will tell it how to read Windows hardware, and the coding will need to allow read/write usage while running the emulator.
As far as the Mac emulator to access the Win drive, I'm "just not a Mac kind of guy".
There should also be some emulators available for this though, and if You want to be positive You won't lose any data while testing all these emulators, You have 2 extra choices.
First = buy a cheap flashdrive for the USB port and test IT with both emulators after they're installed. If You can get IT to format and read/write in both, the haddrive should function fine.
Second = You should be able to install some kind of networking software / network connection between the two systems (Used to be a program/protocol LONG ago called SAMBA) and transfer the hard drive into a NAS case with power supply. This would allow the drives to be used as a "shared network drive" that can be read/written to between the 2 systems through their own hardware connections and if it's an NAS drive it will actually show up on both systems without being attached to either one directly.
They have them every so often but I buy my parts/etc from www.geeks.com for the NAS drive kit.
Good Luck!.