should slices always be referenced with different com ports ?

I wholeheartedly agree with Alan. But to answer your question, you can put the radio into "Split" mode using CAT. When you do this, it will create another Slice, the Split Slice, which will be set as the transmitter. In that case, there will be 2 Slices that are in use by the CAT system.

This stems from a world where CAT supported radios that had a main and sub VFO and Slices are our best representation of those physical manifestation of control.

To Alan's point, this question gets much less complex when you stop viewing it through the lens of how CAT has always worked and start viewing it through a new paradigm — one where you have flexible resources called Slices that can be tuned indepedently and each set as the Transmit Slice at will. This is the power of the ethernet API and it is much less limiting as a result. Some might argue it is more complex. I would say it is more powerful/flexible/capable.

I hope that helps. I would recommend that you refer to the SmartSDR CAT User Guide (https://www.flexradio.com/documentation/smartsdr-cat-user-guide-pdf/) for details on the Split command or any other CAT commands. Feel free to ask other questions as they come up.