Yep. As long as your physical disk is compatable back and forth; that works well.
Thanks for the advice... I understand that the HDs need to be formatted the same to work.
Other than the physical external formatting, you might want to plan your Assets, so that you have the greatest compatibility between the two platforms, also. Some .MOV CODEC's can cause PrPro problems on the PC side of the street, though in general I have few problems with the ones that I get from most Macs. Same can be true for .AVI's on the Mac side.
I've swapped Projects from Mac's to my PC with no issues to date. Only hangup was the FAT-32 format for the external, because of the Mac compatibility (what Curt pointed out). File sizes have to be kept down due to that. Do not know if the newer Mac OS's can read NTSF, but I do not think so. To overcome this, you should be able to get MacDrive for the PC and use a more efficient formatting than FAT-32. I'd check into this, as a 4GB file limit comes up quickly.
In my case, I just plugged the Mac's external and copied the whole Project & media, to my 2TB NTSF externals and didn't have any problems. Never even needed MacDrive.
Ive done it several times with me on PC and a partner on MAC.
In fact once you both have the source files on both machines; you can just email the project file back and forth as you make changes. Its very nice.
One additional consideration will be the drive letter assignation. The Project file (Prproj for the PC) is just an XML database of links and instructions. It contains none of the media, just where to find it, and what to do with it. If you are swapping the drive between two computers, you'll want each to always see that drive as the same drive letter. This is easy on the PC, and is set in the OS's Disk Management. It's bound to be similar for the Mac, but I have no clue where to suggest that you look. In my case I have several 2TB externals that always appear as drives S:\ through Z:\ on all of my machines. This is set in the OS of each machine.
Otherwise you'll spend a lifetime finding and re-linking Assets.