Lets look at it this way.
The client needs 100 items that take 1 hour each to make. That's 100 work hours.
If you had 100 machines the whole job would take 1 hour to do, but still takes 100 work hours. Would you charge that person for only 1 hours work if you used 100 machines ? I doubt it.
Your two machines example is the same as my 100 machines example, except you just have a lesser number.

Answer = You charge 15 hours.

Besides, having the luxury of two machines actully costs you even more than having one machine. You need to cover this cost too.