SQL: How to to SUM two values from different tables

SELECT (SELECT COALESCE(SUM(London), 0) FROM CASH) + (SELECT COALESCE(SUM(London), 0) FROM CHEQUE) as result

'And so on and so forth.

"The COALESCE function basically says "return the first parameter, unless it's null in which case return the second parameter" - It's quite handy in these scenarios." Source

select region,sum(number) total
from
(
    select region,number
    from cash_table
    union all
    select region,number
    from cheque_table
) t
group by region

you can also try this in sql-server !!

select a.city,a.total + b.total as mytotal from [dbo].[cash] a join [dbo].[cheque] b on a.city=b.city 

demo

or try using sum,union

select sum(total)  as mytotal,city
from
(
    select * from cash union
    select * from cheque
) as vij
group by city