Depends on what needs to be produced from example 2 hours.
If something needs 2 hours and 10k WB then you can't use 5k and have it produced in 1 hour...is this what you are saying or do I understand incorrect?
Maybe what you are meaning is that we can use half of the War Bonds needed but with increased times.
10k WB @2 hours production
5k WB @4 hours production