multiple arduino serial communication

I'm trying to get my max msp connected to two different arduino's; sending serial messages by using the max object 'serial a 9600'. Is it possible to send two arduino's different messages, or sending the same message?

With two arduino's connected, my computer chooses one of the two to send it to.

I'm using a macbook by the way. thanks in advance

Please don't post the same thing in multiple places. Plenty of us read everything and will move it if needed.
Please delete one of them.