I'm sort of an achievement hunter who plays mostly singleplayer games (with a few exceptions such as Team Fortress 2) and games that have an optional multiplayer mode. In the latter, there often are achievements that can only be unlocked in multiplayer, and in most cases it would be much easier if you had a partner who would specifically help you fulfill the achievement's requirements.
Since most of my friends either don't play the same games as me or simply don't care as much about achievements, I got the idea to create a second account, use it to download certain games on another device and then join a game with myself to unlock achievements on my primary account.
From what I know, just having multiple accounts isn't against Steam's rules, but I'm still not sure about doing this. Is it considered cheating? Could I risk getting a VAC ban (in games where that applies), or even losing my account? The reason I'd like to think it's okay is that I'd essentially be paying for some games a second time.