Huh, never even occurred to me that might be possible! Do many of you do this?
Are there conditions, eg both must be Steam, or must not? Must you have 2 monitors?
Can you play co-op with yourself?
Back when I played an MMO, I used to run two instances of it so that I could play co-op with myself. I did that regularly in the late game because there were dungeons that were too difficult for just one character. I had each windowed, one on the left, one on the right, so just one monitor. I think if I did that today that I would just be confused and die horribly. I should point out that the game was turn-based, but there was a time limit on how long your turn was, and quite a bit to do for each character. (Edit: Actually you didn't have a time limit per character. There was a single time limit for everyone in the group)
Recently when I was playing an idle game, I left it running for about 3 weeks while also playing other games. Of course, the idle game took up almost no resources.
Yesterday's example with AC and Craftopia was sort of a rarity, where I had two demanding games running at once. However, I had my Craftopia character standing in a small room so that stuff wasn't being rendered on screen. Of course, this mostly just helped my GPU, not as much for my CPU. Then I went to AC and dropped the settings from Very High to Medium. I didn't check my FPS in AC. Was probably not great. But I checked my temps the whole time and they stayed good.
That reminds me of something I forgot to put in the other thread...