It's what happens when you have two subjects, joined in a "koordinativ" way. (I'm not sure what is the correct way to translate this, and what are the correct concepts in English.)
Examples include using:
- sowohl... als auch
- weder... noch
If you have 1st person and 2nd person, then it's 1st person plural:
- Ich und du gehen... (=Wir gehen)
If you have 2nd and 3rd person, it can be either 2nd person plural (more common), or 3rd person plural:
- Weder du noch deine Frau seid... (=Ihr seid)
- Weder du als noch deine Frau sind... (=Sie sind)
We also had an interesting discussion last week about the use of "zu" before an infinitive, specifically, why does it appear when you have multiple verbs, but not for modal verbs.
Also last week (today being the last day) I decided to finally claim that Legendary achievement in Duolingo (which is to finish #1 in Diamond league). All I can say is, never again, even when it was only ~4k XP for me. It's not worth it, I had to pour extra time into Duolingo that would have been better used for other activities.