We’ve actually seen tech like that in action before. Vivo packed it into the Apex concept phone it showed off earlier this year, with an in-display fingerprint sensor that covered almost the full screen and allowed dual simultaneous fingerprint scanning.
That used optical sensors rather than ultrasonic however, so in theory Qualcomm’s new tech should have the edge. Ultrasonic sensors - already seen in Samsung's flagships like the recent Note 10 series using last year's version of 3D Sonic - don't rely on an image of your finger but instead sense the surface texture of your fingerprint, meaning they're both more secure and work even when your hands are wet.
The 3D Sonic Max sensor isn't quite as expansive as Vivo's, and instead measures 20mm x 30mm according to Engadget - less than a third of most smartphone displays, but enough space to fit two fingerprints side by side for added security with the option to scan both simultaneously. Qualcomm says it's aiming for a one in a million accuracy rate, way up from the one in 50,000 that's typical in current smartphone sensors.
We’re not sure what the first phone to feature 3D Sonic Max will be, but with the new Snapdragon 865 chipset likely to appear in flagship phones in the first few months of 2020 it wouldn’t be a surprise to see this improved fingerprint tech make an appearance at MWC 2020 too. Since Samsung adopted 3D Sonic this year, it seems pretty likely to push the new Max sensor into the rumoured Galaxy S11.
Don’t expect it to make it down into mid-range phones for a while yet. When Vivo showed us the Apex 2019 it warned that packing sensors below large chunks of the display was too expensive to roll out widely just yet, so scanning two fingers at once will probably be flagship-only for a while yet.