Probably the easiest approach would be to use two T400 devices connected to a computer via the USB serial interface.
The hard way would be to hack the hardware. There's some breakout headers on the back of the PCB. The available programming space is rather limited, so you would have to disable SD support, LCD, or serial output.
What are you trying to measure? If you are measuring two temperatures and taking the difference, you can actually connect thermocouples in series.
Adding current transformers would be quite difficult and would probably require new firmware from the ground up.
