Well, you could always use a USB extension cable or an external hub to get two dongles attached. I never liked the large size of any of the dongles plugged straight into the pi, and have always used an extension or hub.
That said - with two dongles you’d have to be running two instances of dump1090.
obj gives an overview of how to do that here:
ads-b-flight-tracking-f21/multiple-dvb-t-receivers-and-dump1090-t19896.html
I could be wrong, and you’d have to ask obj to be sure, but you may only get MLAT data from the primary antenna.