Playing two concurrent Characters in Skyrim?

If you're not using separate accounts, quick and autosaves will overwrite each other, manual saves will not (unless you choose to save over another save).

By using separate accounts, the saves will be entirely separate, preventing any of the other person's saves being overwritten while playing.

When you "switch" between characters, make sure that when you save the camera shows the face of your character, and maybe some representative background. This way, you can easily tell to which character the save belongs and reduce the risk of accidentally overwriting the wrong file.