I am trying to build a saved search of customers with a column for Default Billing Address and a column for Default Shipping address. I only want one row per customer, and I want to include customers that have neither a Default Billing Address nor a Default Shipping Address. I can’t figure out what combination of things to have in the criteria/results to ensure only one result per customer. Any suggestions?

It shouldn't return more than one line from just the values you mentioned if you are getting more than one then it's probably another column you're using.

well as soon as I join to address I get multiple lines if the default billing and shipping addresses are not the same

it is regular result of SELECT * FROM TABLEA JOIN TABLEB... you will always get rows "columns A + column B" for each row from tableA and tableB... probably possible to solve that with SuiteQL and nested select