Other cobots, like Yumi and Baxter can drive 2 arms from a single controller so that the arms can be made to move together.  How can one accomplish this with UR cobot that has a separate control box for each are?

@ColorMan it can be done in a couple of ways, you should check out this post: http://dof.robotiq.com/discussion/658/mirroring-robot-movement

@ColorMan do you need the arms to be timed to each other meaning they know where the other arm is or actually have the same motion?  You can accomplish both of these by using the information in the post @Etienne_Samson referred you to. We did a 2 arm proof of concept last year for a customer by having one of the robots act as a master and the other act as a slave.  The customer controlled both robots from a se single controller other than when they needed to manually move the slave arm.  When you pressed play on the master the program on the slave was automatically started for instance.

The master would always tell the slave (via modbus registers) when it was ok to perform a task and then would wait to ensure that the slave performed the task before moving onto the next task anytime the two needed to move synchronously then the two would work on separate tasks asynchronously before coming back to synchronous motion.

Here is a video of the simple POC we did for the customer, very simple motions that are not extremely well synchronized but you can get the idea of what is possible.