Hi Sam57nz,
Before you purchase additional computers ie 1 for each screen, have a "good look" at the youtube videos produced by Michael Brown at the following link https://www.youtube.com/user/col2mab/videos?sort=dd&view=0&shelf_id=0
Michael specialises in computers built for x-plane 10 and the company he operates, X-Force PC, is the recommended supplier of computers by Laminar Research, the developers of X-Plane, within the USA. You may need to view some of his videos as far back as 12 months ago. Having looked at many of these videos I am of the opinion that you dont have to purchase a second high end PC as it can be done on the one machine provided you have additional hard drives. I think the time spent in looking at these videos will well worth the savings in dollars in due course.
I am also of the opinion that he would not produce these helpful videos if it could not be done. Also have a look at his website X-Force PC to get an idea of the specs if you do decide to go for a second machine.
I also understand that MIchael and some of his staff also work for Laminar Research which means they have an intimate knowledge of how X-plane does work.
I also support the comments made by jroberts and this is also covered by MIchael in his videos.
Hope this helps.
Glenn