Multiple Apple IDs to one Calendar

Forgive me if this has been posted before but I did search the forums and nothing came up.
A client of mine has just bought a MBA he and his wife both already have iPhones registered to their separate Apple IDs.
They would like to their calendars to merge in to one central calendar that shows the same appointments on both iPhones and the MBA, also allowing them both to edit the calendar and those changes to be seen on all devices.
Is this possible using Apple's calendar app or is there a better alternative?