Two Mac Minis, Two Mac Pros, One Raid?

Probably not. You'd likely be maxing out the drives performance at that point, with both simultanious reads and writes to the array.

You could probably get away with either writing to the array from both computers at the same time, or reading, but not reading and writing both at the same time, at least not for more than a few seconds untill the buffer fills.

 

Multiple-person collaboration off the same source data like that really requires a SAN, which seems like it's out of the budget for this particular operation.

Not that miniDV is that bandwidth-intensive, but I still feel like four machines accessing one RAID over (I assume) Gigabit Ethernet will be too slow. Really no way to know without trying it though.