I don't see a problem with your plan as outlined. jupo's observation is spot on re power. Didn't occur to me, I'd though it implied by your re-boot at switchover, but nailing that down right off the bat is the way to go.
I've used a
DB25 Manual Switch Box 2 Way Crossover to share part of a SCSI chain and a terminator between a pair of computers. That way they could be running at the same time with their own dedicated external devices (HDD) with proper termination and the chain extension also terminated properly. The box I linked is for parallel or serial so it should be all 25 lines straight thru. I don't think I've ever seen a rotary manual switchbox that didn't have all lines implemented and straight thru, but there may be some out there. Again DON'T do the switchover when the systems are powered up, having everything hooked up to the same power strip/surge protector is a good way to handle that.
edit: even in the situation you've outlined I'd recommend using switchover box and terminator. With the extra cable and switchbox hanging off the backside of the offline computer you should terminate the extra junk.
Here's another box that shows the connectors on the backside with a confusing diagram.
http://iec.net/product/crossover-db25-economy-switch-box/
- computers connect to A/B
- chain/chain extension connects to a
- terminator connects to b
