Both table contain acctnum1 and acctnum2. Acctnum2 is a trailer, a different address for the same customer. For example Barnes and Noble has many stores, their accounts would look like the following
Acctnum1 Acctnum2
12345 0001
12345 0002
12345 0003
All would be Barnes and Noble but different addresses. A combination of the two is a key.

I'd like to do a join on a result set, and I don't know if thats possible.
It would be something like:

select * from customers where acctnum1, acctnum2 in (select distinct acctnum1, acctnum2 from invoices where invoicedate>05/01/1998)

I know this doesn't work so I was thinking I could do a join of the subquery with the customer file. I'm not sure how though.