If you are using the current selection you do not have two ranges. You have one Range with a Start point and an End point. Range.Text is the text between them.
If you are separately obtaining the start and end point by some other means then you could use
Code:
Set rng3 = ActiveDocument.Range(Start:=rng1.Start, End:=rng2.End)
You may find it helpful to review the documentation:
Working with Range Objects | Microsoft Learn
Range object (Word) | Microsoft Learn