Can a Thunderbolt 3 (usb-c style) to Thunderbolt 2 (mini-DP style) adapter work in reverse? I have a mid-2015 Macbook Pro running Catalina (10.15.7). (2.8 GHz Quad-Core Intel Core i7, AMD Radeon R9 M370X 2 GB, Intel Iris Pro 1536 MB)
I have an Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
I would like to drive a small USB-C to Displayport Hub - so I can use two 4K Monitors, but just switch one USB-c cable when switching between my work PC (USB-C) and my Macbook Pro. (I realise I could use mini-DP to DP cables from the Mac to the monitors - but switching machines would require changing 6 displayport plugs each time you change).
I have reason to believe this can work, based on this comment.
My question is: ** Can a Thunderbolt 3 (usb-c style) to Thunderbolt 2 (mini-DP style) adapter work in reverse?**
The Apple TB3 to TB2 adapter is bi-directional, it can connect a TB2 device to a TB3 host, connect a TB3 device to TB2 host, or connect a TB3 host to TB2 host for either to be in target disk mode or to create a network between the two hosts.  What this adapter cannot do is pass DisplayPort signals, for that you need a DisplayPort adapter.  I have the Apple TB3 to TB2 adapter and tested that it will not work with DP devices, at least not my DP devices.
What ports exist on this hub besides the two DP outputs?  Will you be using any ports other than the DP outputs?  There's reasons why it's difficult to find adapters to plug USB-C devices into anything other than USB-C ports.  One is that USB is host/device protocol, as opposed to peer/peer protocols like FireWire and Thunderbolt.  Another big reason is that USB-C can carry far more power than most other computer ports, things can end up not working if there's not enough power and things can end up not working (and on fire!) if too much power is applied.
Generally speaking Thunderbolt and DisplayPort adapters will not care which is the host and which is the device.  Thunderbolt doesn't care because it is a peer/peer protocol, the cables are usually active but there's no distinction between input and output ports.  DisplayPort cables typically do not care because the cables are passive, they just mate one port type to another with no changes to voltages or whatever.
If all you care about is connecting DP displays to DP hosts then you can likely find inexpensive DP adapters to do what you want.  If this hub is used for anything other than DP devices then the solution will be either impossible or expensive.