Are you looking to do a "quick search" for a text query across multiple sites or are you wanting to query across sites using JQL / "advanced search"? This isn't possible at the moment but I'd love to learn more about what you are trying to do.
I've found some docs that can help you link an issue on one site to an issue on another: https://confluence.atlassian.com/jiracorecloud/linking-issues-779296002.html
Product Manager @ Atlassian
Hi Atlassian Community, My name is Avni Barman and I am a Product Manager on the Atlassian Access team! One of my top priorities is to help make the administrator's life easier through improved pro...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events