If you actually want to _share_ the media (using the media on two systems at the same time) you’ll need some sort of SAN, like Unity or Terrablock or Editshare.
Other than that, if you need to move a media drive from one to the other, you’ll run into the issue that Macs need HFS+ formatted drives, whereas PC’s use NTFS formatted drives. Both can use FAT, but those are far from ideal for media storage.
The solutions I know are MacDrive on the PC (will allow your PC’s to read & write to HFS+ formatted drives) and Paragon (will allow your Macs to read and write to NTFS formatted drives).
I’ve pretty intensively used MacDrive for a few years, and that works well. I’ve only used Paragon occasionally, and found it disappointing, but that might have had to do with the type of drive I was trying to mount.
Either way, these solutions are not really supported for use as media drives on Avid systems. I can say that I have used HFS+ drives on my PC’s for Avid media (usually offline resolutions) and that worked fine. However, OMFI MediaFiles folders can get pretty crowded and having more than 3-5000 files in a folder will definitely slow down finder performance when trying to open such a folder on a PC with MacDrive. Since the new file format, MXF, has a different folder structure (limiting the amount of files per subfolder to 5000), that would be a better option.
Also, you might find that if you move the media from a Mac to a PC or vice versa, that the system will want to rebuild the media databases. On large projects, that can take considerable time.
Practically, the files and folders themselves are completely exchangeable between platforms (although older file formats like SD2 are limited to older Mac systems).
I’ve actually had much better luck using Paragon NTFS on the Mac side than MacDrive on the PC side. I have a 12TB RAID hooked up to a MacPro via Fibre Channel, formatted as NTFS, and I get 300MB/S Read speed (lower write, but fast enough for my needs). Media Composer has had no problems so far working with this setup (though this is primarily a Final Cut system – Final cut has had no problems either).
Performance with MacDrive has been fairly bad – good enough for moving files around, but not good enough that I’d want to rely on it for a working drive.