Spend money on pay, not supply
The boards are already in potential crisis, and the idea that serving teachers take on second batches of marking and claim a fortnight off school will not work. People willing to take on double loads of marking are unusual, the results are often unsatisfactory, and the administration would be complex.
A reputable public exam system will have to cost more, but I think a better way of spending the money would be to increase the pay per script to the point at which an hour's marking could be reckoned to be worth an hour's teaching. This should be funded by government, rather than increasing entry fees.
Serving teachers could then be offered a choice of either heroically combining marking with their day job, or taking a statutory entitlement of two weeks' unpaid leave for external marking. This could legitimately be regarded as professional development. They would earn at least as much as in the classroom while having a change of activity, and their schools would have the money to pay for supply.
There are ramifications to be sorted out but this model could be one way out of a very serious problem.
Andy Connell 1 Mill Hill Appleby, Cumbria