You will need a microphone mixer to accept two microphones and mix the output down to a single output to feed your camera.
Make sure that you get microphones compatible with the mixer that you choose, some of the better mixers may have only XLR type microphone connectors which is what you need if you have longer mic cables because these use low impedance balanced lines to minimize electrical noise pickup but the microphones are going to be more expensive. But for typical use, you will probably want a lower cost alternative.
You are welcome and yes it will mix the two mics to a single audio track allowing you to vary the level between the two mics to get what you want.
But depending upon what you need to accomplish, you can try a very simple and cheap "Y" cord which has no electronics. It will have two female jacks connected to one male plug allowing you to plug in two mics and then plug into your R800. This simply runs the two microphones in parallel and it MAY work for what you need. You can't vary the level between the two mics and for best results you will want to use two identical mics and they should be of the type that doesn't require external power in order to be compatible with your camcorder.
The cord option is inexpensive and if it sounds like it may work, try it first. You can do a google search for: 3.5mm stereo y cable dual female and it will show choices for under $5.
For the USB mics, you will need a mixer box that takes USB mics so the cord solution won't work.
I don't think this forum will allow Amazon links and they wouldn't be my first choice for cords because a lot of their stuff is true junk. Do the google search making sure that the cord you choose has a 3.5mm stereo male which will plug into your camcorder and it has two female jacks which will take standard 3.5mm microphone plugs. Before Radio Shack went away, they were a good place to pick these sorts of things up.
It appears even Walmart stocks these in there stores, they call them a headphone splitter but they will also work the other way to feed microphones to your camcorder. Just grab the one with the correct size plug and jacks. Best Buy likely also stocks these if there is one in your area.