is it okay for actors to interact when competing for a role?

I am running some screen-tests and I have two actors right for the lead role but one of them is also right for a side role. I want them to perform all scenes together. Should I do this, or carefully schedule the times so they never meet. But then again I want to see how they interact on screen.
 
They should be happy with it... if they're not, they have flawed logic. If they refuse, one gets the lead and the other gets nothing. If they play against each other, they potentially both have parts. Is this a paid gig or Copy/Credit?
 
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