Having two accounts from the same household is fine. In fact, you can vote on each other's posts if you come across them, but you should be sure to vote only based on content, not based on who made the post. And you shouldn't intentionally look for each other's posts. This is covered in What is the procedure for having my spouse/significant other as a member of the same SE site? If you're worried about being impartial, then just don't vote on each other's posts at all.
In fact, even one person can have multiple accounts. In this case the connection should probably be disclosed, and the accounts most certainly should not vote for each other. I've seen this done where, for instance, a high-rep user or moderator wants a less-privileged account to suggest edits or closures, rather than force them.
As for logging out, look to the top bar, on the left:
There is a logout option in the menu: