Read them separately (and privately please! ) Give each actor a direction to see how they take it....whatever it is you hope to see. Should tell you what you need to know.

I usually have them come in and read for an entirely different scene from the play.  I give them some specifics about what I want to see.  When they arrive, I have their partner be the actual actor I've casted in the opposite role (i.e., Don Lockwood against a Cathy Selden).  When they audition at callbacks, I have them work a few times with the actor.  I see what the chemistry is like; I observe how they take direction; I listen to their speaking voices--are they interesting to listen to? do they vary in pitch a great deal? do they have a feel for the rhythm of the script?.  I sometimes even have them do a pantomime, watching for the physical acting.  If it's really close, I'll have them work a few improvisational exercises to determine which one has the best working brain--which can make or break a performance.

If possible, I don't formally cast the rest of the roles of the same gender.  If I am dealing with just two different actresses or actors, I have them wait until a decision is made.  I ask for input from stage manager and the other performer who worked with them--although it's only input, not a decision.  I ask for specific reasons to back up the blanket statements.  Whichever one doesn't get the role, I'll give them the choice of the roles I think they're best for and allow them to choose which one they want.  If I have someone that is that good, I always want them onstage in some role!  I also offer them the chance to understudy the role they didn't get.

Wow!  What great techniques!  In the CT that I work with, we usually don't have the privelage of having multiple actors/actresses that audition who are so great they have to come in for second round auditions.

But if the situation ever arises....  wouldn't that be cool!