I feel like this is a really win win situation. The person with the job feels at ease and the person who's camera is on the shelf has the possibility of making some money.
Question is what is a fair and reasonable percent to give the back-up body if it end up working on the job? What is a big enough incentive to be on-call and willing to fly in the body if needed?
Im trying to see it from both sides.....
EDIT: things to consider:
1. The rate needs to be discussed beforehand. Obviously we all want 1k/day for our kit but really that is rare.
2. Firmware needs to match
3. ETA on an emergency delivery should be established the night before - as we all live different places and shoot different places too.