For the PC version, if you start a new game does that overwrite the old? My brother and I would like to play on the same computer but can the game support 2 separate games?

Yep, the game tracks each character made, you can even choose when you load a save to filter which characters saves it shows.

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forget what I typed.. I was using the old FO3 / Skyrim method.. use this ^

On PC you can use the console command line to save games with specific names:

save GAMENAME

This saves the game under the specific name you give it instead of "Commonwealth" or wherever you are.