What if a sub element is used by two other independent elements when using RELAX NG Compact?

I am using RELAX NG Compact and have run across a case where a sub-element is being used by two independent parent elements. How can I resolve this?

Use case 1

<parent1>
  <field usecase_123="test" />
</parent1>

Use case 2

<parent2>
  <field usecase_AAA="test" />
</parent2>

Herein lies the conflict:

parent1 = element parent1 { element field { attribute usecase_123 {text} } }

parent2 = element parent2 { element field { attribute usecase_AAA {text} } }

RelaxNG allows an element to have different attributes based on its parent element.

So you can have a RelaxNG grammar like this:

start = root
root = element root { parent1* & parent2* }
parent1 = element parent1 { element field { attribute usecase_123 {text} } }
parent2 = element parent2 { element field { attribute usecase_AAA {text} } }

And, valid against that grammar, the following document instance:

<root>
  <parent1>
    <field usecase_123="test" />
  </parent1>
  <parent2>
    <field usecase_AAA="test" />
  </parent2>
</root>

…while, invalid against that grammar, the following document instance:

<root>
  <parent1>
    <field usecase_AAA="test" />
  </parent1>
</root>