The other email is optional, you can just skip it.
Discussion
Have you tried recently? Won't let you create an account, at least that was my recent experience on both services
This is from 5 mins ago 
Did you click on maybe later? It is great they added that if it really works, I will try again and create a new account.
Few months ago was helping a friend setup an account, impossible without mobile or email verification. I reach out to Proton and they stated it was for security to avoid spam and that they did not keep that information... sure... tried tuta, the same... so had to use a disposable email to continue