Integrate custom calendar into 2 separate websites?

We're going to build a customized web calendar control for our company & their employees.  The catch however, is we need to implement this control into 2 of our company websites.  There's a lot of work going into it (about 300 hours total), so we'd like to build it once & have both sites be able to utilize it.  In addition, we'd like to set it up so that if we make a change to the control, both websites automatically reflect that change.  No need to re-update both sites.
What's the best way to set this up?
Thanks