I have been fiddling with tracking as it is the only way I can get the distance sailed as the odometer doesn't seem to work. Seems to me that internally it uses just ONE file for track recording and it needs to close it to export it. It creates a new track section for every time you stop/start tracking which I can see in the exported GPX file.
If you import a GPX file it will just add its content to its single track file. I have started to export them, then break them up into meaningful sections. Erase it on the TZT2 and import whatever I want to have visible on the display.
I have two TZT2's and they record track entirely independent so that you can have different track settings, like time or distance based. Unfortunately it is very difficult to get at the USB port on one of my displays and there seems no way to exchange track files between displays.
I also noticed that tracking may not automatically resume if it was on when you had to reset the MFD. One of mine was frozen and the only way to revive it, was to press the power key for a longer time. After that, it didn't resume track recording and it took me a while to realise it. Luckily I had the track from the second MFD, but had to dismantle the dashboard to get at it.