We moved your post to the "Servo controllers and servos" section of the forum since it seems more appropriate.
How you connect the two Maestro controllers will depend on what control interface you are using. For example, you could control them from USB via a PC by connecting the 12-channel Maestro through USB and leaving the TX and RX connections you described. If you are using a separate microcontroller (like an Arduino) to send TTL serial commands to both Maestro controllers, you would connect the TX pin of the microcontroller to the RX pin on each Maestro, then connect the TX pin on the 6-channel Micro Maestro to the TXIN pin on the 12-channel Maestro, as well as the TX pin on the 12-channel Maestro to the RX pin on your microcontroller.
Another option is to have the 12-channel Maestro run a script that controls everything. In this case, you can simply connect the RX and TX pins as you described. This way, the 12-channel Mini Maestro can send serial commands to the 6-channel Micro Maestro to control the additional servo channels or run predefined subroutines. The Maestro's scripting language is based on Forth. The Maestro Scripting Language section of the Maestro user's guide has several examples and some step-by-step walkthroughs, as well as a list of valid commands. If you are new to this kind of programing language, you might consider loading one of the examples onto your Maestro by copying it from the user's guide and pasting it to the "Script" tab in the Maestro Control Center software and clicking the "Apply Settings" button. You can do this with no servos or other peripheral devices connected and use the "Status" tab to see what the servo channels are doing. There is also a "Step Script" button at the bottom of the "Script" tab that allows you to run a single line of the script at a time, which is very useful for troubleshooting code or understanding what each command is doing.
If you are still unsure on your wiring and connections, you can post a purposed wiring diagram and I would be glad to take a look.