how to show combined results of two tables?
Test

SELECT t1.InvoiceID, t1.UserID, t2.InvoiceID, t2.UserID
FROM ( SELECT InvoiceID, UserID, @num1 := @num1 + 1 rownumber
       FROM au.invoice, (SELECT @num1 := 0) variable
       WHERE InvoiceID &gt; 1234
       ORDER BY UserId ) t1
JOIN ( SELECT InvoiceID, UserID, @num2 := @num2 + 1 rownumber
       FROM nz.invoice, (SELECT @num2 := 0) variable
       WHERE InvoiceID &gt; 1234
       ORDER BY UserId ) t2 USING (rownumber)


fiddle

If the list of UserID values in both subqueries is the same with guarantee (although it is unclear why) you may use JOIN ... USING (rownumber, UserID). Or even use the enumeration while partitioning by UserID (using 2 variables in each subquery).