How do I see everything we've invoiced one customer across companies?

Accounts → Customers (/accounts/customers): every sales invoice folded into one record per customer, most recent invoice first, fifty a page. Each row: invoices, invoiced, outstanding, last invoice, and how many Xero organisations bill them (two or more is a customer billed from more than one company). Search by name, VAT number, company number or email; click a row for the customer card.