Let me elaborate a bit on this issue.
What we are trying to do here is to find for a user action BT the source of the visit (i.e. referrer).
So we built 2 BT's one of the visit split by the referrer and the other one on the inner user action and now we want to match the 2 BT's with the session id.
Our question is if there is a way to create a measure of this session id in order to split both BT's by them?.
I dont think there is a VisitID Measure in the moment. I see why this could be useful in your scenario especially after you gave me more details via email.
As I know some of the details of your enviornment I think another option would be that your developers create a unique ID on the browser and also send that on each Single Page Action. That could then be used for your efforts to "split" all these User Actions and to then "group" them together
I will forward this to the product team though to make sure we do not have any internal VisitiD measure. But I think it doesnt exist. Could be a good Product Enhancement as well