Can two people be logged into the same psn plus account at the same time and play different games online?
No, this isn't possible. The moment the second person logged into the same account, the first person would be forcibly logged out.
This has happened to me with my friends who have PS4s I've logged into in the past. It will kick your friend offline even if you're not playing online, too. (I was kicked out of a Dark Souls III boss when my friend was trying to do local multiplayer with a friend in Rocket League with my account)