First you need to find out "the join" between the two tables (how do you know which cities a customer lives in).
Lets say you have a field in customers table named cityid that points to field id in citytable, you can do this without changing customers table:
select cust.* /* here you should write each and every field you want as a result */
from customers as cust
inner join citytable as city
on city.id=cust.cityid
where city.city in ('Paris','London')
;