Hi there, I have my main Steam Account which I use daily, and have done for over a year now, but I also have an account that I used over the year ago which is now a kind of back up account, I have games on there that I would like to play, that I do not have on my main account, however if I was to sign out of my account and into my old/back up account, what would happen, would I be able to keep my games on my main account downloaded and be able to download my other accounts games also? I'm not intending on playing my main account's games either, just want to play a few off my old account,
You would only be able to play the games on your backup account, Steam won't let you play games which were purchased on your main account.
I stated at the end of my question I was not intending on playing them, I want to know what would happen if I was to sign out of my main account into my back up account and download it's games, and then after go back onto my main account, not at all am I intending on playing each accounts games on the other account, I'm aware that does not work but I also do not want to do that.
Go into your settings and turn on family library sharing in the family tab on that computer with both accounts that way you would be able to use the games from both libraries on both accounts on that machine as long as you are not logged into that account on another computer playing a game.
As far as I know family sharing is live (I was/am in the beta for it)
Besides that to answer your main question nothing would happen to the data.