Not possible without resorting to hacks. If you had a UR44C, you could connect Line Output 3/4 (Mix 2 output) to your PC sound card’s line-in and send only the mic to Mix 2, while muting it in Mix 1’s Loopback.
If you don’t need to use another ASIO application at the same time, you can use ASIO-Bridge to expose the microphone input as a WDM recording device and use it directly in Windows, while muting the microphone in Mix 1 so it’s not part of the loopback.
You could also use an ASIO-aware recording application which can access each input independently - just mute the microphone in Mix 1 so it’s not part of the loopback.
Unless you need the microphone to be routed through the ASIO interface for some other application, you’re better off connecting it directly to your PC sound card for most desktop applications.