I have two Steam accounts with a variety of games. One was when I was younger that I lost access to and recently got back and the other is one I've used until now. Potentially, is there anyway to download Steam to one hard drive and log into one account there and then download Steam again to a different hard drive and sign into my other account on that one? I just really hate signing out and signing back in every time I want to play something else.
You can't have Steam installed multiple times on the same machine. However, what you can do is enable Family Sharing between the 2 accounts so you always got access to most games on both accounts.
How do I enable Family Library Sharing on my computer?
To enable Family Library Sharing, first be sure you have Steam Guard security enabled via Steam > Settings > Account in the Steam Client. Then enable the sharing feature via Settings > Family, (or in Big Picture mode, Settings > Family Library Sharing,) where you'll also authorize specific computers and users to share.
This won't give access to all games, because the publisher can decide to disable family sharing.