Is there a way to control a cortex using another cortex?

(This is not VRC legal)

Yes! As @jpearman demonstrated at one point, you can communicate between two cortexes using I2C. See is post here for more information

Thanks! But don’t worry, this isn’t for our competition robot.

Sure, just depends on what you have in mind. You can interface between two cortexes using the UART ports, then run a controlling program in one and a controlled program in the other. What you send from one to the other is entirely up to the programs. (as I was typing, @MayorMonty’s message came in. Looks like I don’t need to link to @jpearman 's stuff; you’ve got it now.)

I’ve also helped do it slightly differently. Using a logic-level to rs-232 level converter, we hooked the UART port of a cortex to the partner joystick port of a joystick linked to another cortex . Then, the cortex can pretend to be a partner joystick, sending the joystick message (which includes information about all the switches, potentiometers, and accelerometers on the partner joystick) periodically. Then, the receiving cortex can respond to the partner joystick inputs in any way you like. In reality, you aren’t limited to processing the partner joystick message as joystick control information. You can use this for general purpose message passing, though it’s only in one direction.

6135K used it to control a robot remotely from unusual sensor inputs. Here’s their Future Foundation Robot Construction online challenge video about it:

And here’s the brief write up from the entry: