Currently, JQL (Jira Query Language) only works within a single Jira site (instance).
You cannot write a JQL query that fetches issues from projects located on two different Jira sites—even if both sites are under the same Atlassian organization.

It will be good to have such ability to perform JQL search for the work items on different sites in same org.

Workaround:

Run the relevant JQL on each site separately.