Here's the big problem with that idea: the stock data is shared among all windows in MT. That is why it is problematic to have two sources get data for the same symbol - it clashes. In the case you describe, the big port, after it creates the little port, will continue running the scans on the stocks in the little port if the little port is connected to a real time source, and any data that will come in big port will conflict with the data on the same symbol collected by the little port.
But - what we *could* do is allow a button to create a portfolio or a watchlist from the results of a scan on another portfolio - manually. Then you could conceivably open a big portfolio/scan, run that preliminary scan on it, generate another portfolio from it, turn off/close the big portfolio and place whatever scan you want on the newly generated one.