I guess you could just add a third expression? I'm really not sure what you're asking at this point, it would be much appreciated if you could clarify your question
Sounds to me you need to make a field [Time Type] that has 'Late Time' And 'Start Time' in it and a field [Time Value] that contains the values. Use [Time Type] as a dimension and [Time Value] as a measure. The Session dimension can be used as is.
