Each UART is compatible with either the legacy driver (producing naming convention “ttyS#”) or Tegra High Speed driver (producing naming convention “ttyTHS#”). Typically (probably not a guarantee), if it is the same “#” for both, then it would be the same UART/pins. Just don’t use two drivers simultaneously on the same UART.
I also suggest using a serial console program, e.g., gtkterm or minicom to test for echo in loopback since it is more likely to have all settings correct, and to also give an easier way to change settings.