Yes, staves can be too dry to make bows. If they are stored in a safe place where insects, etc can't get to them they should be fine even if you are away for 2 years. When you get back, check the M/C. On most woods I am familiar with, 9% to 11% is what you are looking for. Hickory can be 6% but we aren't talking about hickory. If the wood is too dry it can be re-hydrated by leaving it in your bathroom or other area of high humidity for a while.
You can shape a bow from wood that is too dry and get it to floor tiller stave without worry but be sure to get it to the proper M/C before stressing it at all. Pat