How to cross acocunt access an AppSync API from another AppSync API?

I have two AppSync services in two accounts, A and B. I'd like to call AppSync A from AppSyncB. AppSync B is protected by IAM auth. AppSync A has an HTTP data source to make requests to AppSync B. I created a role in account B that permits all API actions on AppSync B.

When I pass role B to the HTTP data source in account A I get this error.

Cross-account pass role is not allowed. (Service: AWSAppSync; Status Code: 403; Error Code: AccessDeniedException...)`

Then I created a role in account A, with permission to assume the role in account B. I added this role to the account A data source. I still get access denied exception when trying an HTTP request.

Any ideas? Is there a better recommendation for how to architect this?

Thanks