You only need one Captain Crow for Zenith Dragon. It's Rigor Mortex that needs 2. So, if you've synthesised away your Captain Crow, you're SOL until you scout another one using Tag Battles.

If you do have Captain Crow, the Recipe for Zenith Dragon (Quadrilineal Synthesis) is :
Saggitar + Dracolord + Xenlon + Nokturnus

A word of warning though, the synthesis process is a long an involved one that requires a lot of patience because to get Sagittar, Dracolord, Xenlon and Nokturnus, you have to synthesise a whole bunch of other monsters. Use this www.woodus.com/den/games/dqm5ds/synthesizing.php
to find out what monsters are required for which synthesis. Also read the check box properly to make sure that you get a recipe. A lot of people don't read the check boxes for the search function and end up with no results whatsoever for their search.

Good Luck