if the numbers 'identify' the rows, so number 1 in both table refer to the same 'couple', then you can do outer join.
In SQL it would be select * from table1 as t1
left outer join table2 as t2
on t1.number = t2.number
where t2.number is null
this is return all rows in table1, and any matching rows from table2. If there is not a match in table2, then the value is NULL.
you can link tables in CR, and there is the ability to do an outer join (right click on the link line). I don't know how CR determines left and right.
without the link, or using an inner join, then the formula that you propose won't work. With link, it would be ISNULL(Const.Spouse) (because it returns True if the spouse is missing ie null, and false if they exist)
HTH