XSD “one or both” choice construct leads to ambiguous content model

Question

I am trying to make a simple XSD choice construct allowing either one or both of two referenced elements, but not none. The construct is similar to below but I keep getting an a

Answer 1

Some additional hint, if you have multiple elements linked and you want one bundle of elements or the other bundle, or both, you can do it like this:

Just in case some of you bump into the same issue!!