You are making some fundamental mistakes here.
There is no difference in the number of R/W activities, since you perform the same actions. The only difference is how you distribute them.
Your two disk example means reading the media files and writing your export files to the same disk and disks can only do one thing at a time. In your three disk example there are the same activities, but one disk for reading and the other for writing.
And your calculus is off. How can the same activities lead to 1x write with only 2 disks and 2x write with 3 disks?
To put it in another way:
One hour of DV material, around 13 GB that needs to be read and written to the same disk means a transfer of 26 GB (13 read and 13 written). Spread it over two disks and each disk only needs to handle 13 GB. Average performance gain close to 50%, disregarding overheads.
Thanks for the response.
"...disks can only do one thing at a time" -- this explains a lot. All this time I was thinking hard drives have separate read and write heads.
I'm not sure if there ever were separate read and write heads on disks, but it would make the failure rate go up, the cost of the disks skyrocket and the mechanical wear and tear go up, so it makes sense not to do that.