You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an
alternative browser.
Simply put - somehow make it possible post-1066 for the same man to be
A) The Vice Gerent of Christ (of at least one major Christian faith)
AND
B) Caliph of one of its major Islamic contemporaries.
Bonus Points for
1) Byzantine Emperor holding the titles.
2) Maintained and acknowledged till the present.
Have fun!