Since you are using a Laptop, and ingesting i would assume (c920) at 1080p... you need to ensure your USB on the laptop is duel channel or is USB 3.1 v2. As USB 3.1 V1 cannot ingest dual 1080p streams.

If you do find your laptop has duel channel USB 3.1 v1 then ensure both cameras are on different channels.

i would venture to say that the c920 cameras are just that, low quality 1080p cameras. the quality you will see on vMix using them over USB is the exact same you will find ingesting them over a much improved USB connection or capture card.

I use to ingest two of these as well, and the quality is just poor.

My suggestion is to buy a DSLR camera. I now use a single DSLR Mirthless Panasonic G7 and its view angle is so wide I can clip out several virtual inputs from it to show different areas of my room.

to summarize, C920 are low quality 1080p. USB 3.0 (3.1 v1) can do all those need for two, as long as you have duel channel USB on your laptop.

Side note. if you are using any graphics card processing and using a graphics card lower than the 1660 from Nvidia the encoder is not great. i would then suggest you move to x264 processor encoding using the slowest setting your CPU can handle. this will increase the stream quality, not the camera quality.

Take a look at the vMix reference machines (the ruby) and use the cards they offer offer it. your best bet.

You can use a Nikon D750 if it is able to stream its video over HDMI. you could use an Elgate Cam Link 4K for it. they are cheaper option than a real capture card. Your PC would see it as a webcam