Editing start stop locations

I have imported a large live concert into Ardour inside Mixbus 32C, I successfully exported the first song mix, but now I can’t edit the start stop locations to the second song. This seems like it would be easy to work around, but I have not found a way. Is there any help here for this problem?

Instead of using start and stop markers for this, why not define a range instead and export the range? That way you can have ranges defined for each song simultaneously in case you want to go back and work on a previous song.