You don't need 2 TIUs. The TIU's channels are isolated. You can use Fixed channel 1 for loop 1 and fixed channel 2 for loop 2. Power them from your Z4000 as you describe. You don't need to use super TIU mode.
My setup is pretty close to yours with two isolated loops and 2 TIU channels. Main difference is power, I am using 2 PH-180s, one per channel. I am using a Z500 (from an MTH set) to power my TIU through it's Aux power port.
