I recently just cast my graduate thesis film (a musical comedy) and doing chemistry reads was certainly part of it. IMO, as a director, I try to maximize efficiency, and I don’t want to make the actors work more than they have to. We handled it a mix of two different ways:
We had our lead cast early, so we had her in the room with us as we were first-round auditioning her counterpart. That was helpful because we could immediately put scenes up on its feet and watch how it goes. Honestly this was a little bit of good luck too–our lead was the first person we saw on the first day and no one else who came in for that role met the bar–so it helped out casting her partner.
There were mom and dad parts that we needed to cast, and there’s a duet number with the two of them so we knew the chemistry here was important. I did a first round of calls individually with each actor. Had to make sure they could both act and sing in their own right. Then, for callbacks, we went through the tapes and set up “pairs,” and brought them in like that. In this case, one of the pairs was perfect and we went with them–but there have been prior times where after callbacks like that, I casted two people from two different groups.
I would be weary of holding actors for a long time in order to try the different combinations that you need for your ensemble, especially for the first round. Callbacks are an important part of the process, and that’s where it’s kind of expected that we’re going to play around with different things. I think it’s helpful to record everything and then use those tapes in your own debrief afterwards to try to match people up. Also, choosing good sides (not necessarily sides from your script) that best articulate a character’s “stuff” is important.
Hope this is helpful!!! I realize this is a student project (as do the actors you’re bringing in), so if you keep it chill, light, and easy-going; it should be a good time no matter how your process goes. Above all else, making it a not-awful experience for everyone involved is your #1 priority. If you foster a great environment for your crew and the actors, you’ll get the stuff you need!