Setup multiple "monthly" or "annual" subscriptions, such as Pro v Plus plans, each having the same time periods?

I have tried searching, but I wasn’t able to find a clear answer.

I want to have a custom experience for a feature available within our web app only: Pro & Plus plans, each with monthly & yearly time periods.

So far I have:

• added my products to the product-catalog:
  • widget_annual_pro
  • widget_annual_plus
  • widget_monthly_pro
  • widget_monthly_plus
• Then I have created 2 new entitlements:
  • widget_pro
  • widget_plus
• Then I try to create an offering,
  • I successfully get the first “Monthly” product, “Plus”.
  • but I cannot select more than 1 “Monthly” time period to add “Pro”:

    • 

So, how do I create the offering?