So you are doing TWAIN scan (Application controlled), not Scan to PC (Xerox / User controlled).
TWAIN drivers cannot pick both, they need to do one or the other, and since you already have to go to the printer and put the paper in the feeder or on the glass, there should be no surprises. Due to this limitation, Microsoft uses profiles so you can have dropdown options. To create them you simply go into Scanners and Cameras > Scan Profiles > Add and make one for Feeder and one for flatbed
Set the one you will use most often as the default
Then in an app you should have the one you want as default at least. But there is no way to have both set as the source.
Xerox Scanning (Done from the printer) will feed the paper from whichever source automatically, identical to copies. TWAIN/WIA is simply unable to do it.