Do you have a USB microphone and a USB guitar interface? No, In general, Windows only allows you to select one recording “device” at a time. There are “work arounds” but I don’t know if those work-arounds work with Audacity, and from what I’ve read it’s tricky and it’s a pain and it’s never worth it.
Yes, you can use a mixer into line-in on a desktop/tower computer or you can use a mixer with a USB connection (with one input panned left and one input panned right).
The most common solution would be a USB audio interface[u]example[/u]). Interfaces with switchable mic/line and/or mic/instrument inputs are common. If you buy an interface, I recommend getting one with zero-latency hardware-monitoring. There is always some latency (delay) through the computer, and although it’s often possible to get the latency down to where it’s not noticeable, it’s best to avoid the issue completely and monitor directly through the interface. (Of course, you can always monitor from the output of a mixer.)
Note that “audio interfaces” (and mixers) have balanced connections for stage/studio microphones and they don’t work with “computer microphones” or USB microphones.
multitrack USB port recording
Note that “multitrack” usually means more than 2-channels. There are lots of audio interfaces with more than 2 inputs, but Audacity is not the best software to use for multi-track recording. And normally, you’d be multitrack recording from one “device”.
Most USB mixers send the 2-channel stereo mix to the USB port, but there are some higher-end USB mixers that allow multitrack recording (with the appropriate DAW software).