Why you would ever need two CIM's into two stacked toughboxes, I do not know. You are lifting, something easily done by one CIM fairly quickly.
While I have never tried it, my team had a single CIM feeding a DeWalt feeding the guts of a Toughbox (re-made side plates and new input gear, same gears, bearings, and the long shaft with a bearing block on the other end), and after about 6:1 chain reduction we flipped ourselves over using the vertical pole. That is a lot of load. And it was happy. Our math says the end joint (with 6:1 chain from the toughbox output) had around 450 working ft-lbs of torque. Stall on the gearbox output is around 180 ft/lbs. It lifted the robot and held it while it wiggled (the DeWalt had anti-backdrive pins). Here's a pic of the prototype:
http://www.chiefdelphi.com/media/photos/34788
Because both the DW and Toughbox have around 12:1 reduction, this would subject the toughbox in this pic. to around as much load as the second in two toughboxes (but this only has 1 CIM). We made sure in software that we never stalled, but on the one rare occasion that it happened, the CIM began to get.. unhappy. Had we run it longer, it would have smoked before anything mechanical gave. So, with one CIM, you will be fine.
The toughbox is... tough.
I am a little confused on your setup. Are you asking to run two CIM's into one toughbox, then the output of that into another (as I assumed above), or something different.