[NAME]  Sorry I didn't see this earlier.  The Cable Matters MST hub does support dual 4K 60 Hz, but only when paired with a system that supports DisplayPort 1.4/HBR3 over USB-C.  The XPS 13 7390 2-in-1 does because it uses a Core 10th Gen CPU that uses Intel's "Ice Lake" architecture, which was a complete redesign that included a new GPU.  But the regular XPS 13 7390 uses a Core 10th Gen "Comet Lake" CPU, which is just another minor refresh of an architecture that began as Broadwell (Core 5th Gen), and which therefore includes an older GPU that only supports DisplayPort 1.2/HBR2 over USB-C.  That only offers enough bandwidth for a single 4K 60 Hz display.  If you want to run dual 4K 60 Hz from a single port on your system, you'll need a Thunderbolt 3 to Dual DisplayPort adapter.  It looks pretty similar to an MST hub, but since it uses TB3 it can tap into two full DP 1.2 interfaces rather than just one, which is enough for dual 4K 60 Hz.  Sorry I don't have better news for you, but at least you've got the answer!
I have the xps 13 7390 (i5-10210u, UHD 620, not 2-in-1) and two Acer Nitro VG280K (4K[NAME]) monitors. I have a USB-C Displayport MST hub (Cable Matters; amazon asin B07XYCHGQM) hooked up to one of the thunderbolt ports on the xps 13. The two monitors are connected to the hub via displayport cables. With this setup, I get up to only 4K[NAME] on both the Acer monitors. Even if I close the laptop lid (so that there are total two displays), I still can't get to 60Hz on the Acers. If I remove the hub, and directly connect the Acers to two ports on the xps (via two usb-c to displayport adapters; amazon asin B07YDNSGY2), then I get 4K[NAME] on both monitors. The Cable Matters hub is advertised as dual 4K[NAME] capable. What am I doing wrong?
XPS 13 7390 (not 2-in-1) support for dual monitors at 4K@60Hz