I often co-direct with my wife, which may be a different dynamic than yours, but I think there may be a couple good points I can offer.

First - Play to your strengths.  I'm a good "overall concept" and "rhythm of the show" type of guy.  My wife is a detail fanatic.  I'm obsessed with sets, she is the same about costumes.  We work well together because we tend to fill in each other's blank spots.  If we ever disagree on anything, we talk it out (outside of rehearsal) and come to an understanding.  Sometimes my view will prevail, sometimes hers.

Second - You absolutely must speak with one voice to your cast.  One of you should be the designated "speaker."  The cast has to know who to listen to.  I've acted in shows that were co-directed and received conflicting directions from both directors.  (This is where it's extremely important that you each understand what you're going for with your production.)  So, for example, one person should do all the talking for blocking purposes (although both of you are sitting there).  Then, when it comes to notes, each person gives his own set after each scene.  You need to demonstrate to the cast that both sets of notes carry equal importance.  And never, ever let your cast see any conflict.  If the two of you disagree about something, work it out in private and then speak to the cast.

It's like being effective parents.  You have to present a united front, otherwise some in the cast will play you off one another and chaos will not be far behind.

I suppose it all just boils down to two things: "Communicate, communicate, communicate;" and "United you stand, divided you fall."

Anyway, that's my two cents worth. - - - Break a leg!!!

edh915...thank you for all your helpful advice.

I wouldn't co-direct if you paid me a million tax free dollars or put a gun to my head. 

Many directors I have worked with block everything out as they go.  Like everyone else they have clearly read the script more times then they can count and have an idea in their head.  Complex scenes sometimes are written down others are just blocked as they go.  Sometimes seeing it as you go prevents you from staging too many scenes stage left to many up stage, etc.  I kind of like that approach, but for co-directing I will be doing it before hand.

I agree with B-M-D  ....................  The gun to my head part.