I know if a revocable trust fits the definition of disregarded entity that it can use the grantors TIN. If there are two grantors how is it decided on which to use? We have a customer that says that the trust has had no income and they are wanting us to use both.

Every account is tied to one SSN/EIN. How can you have two assigned to a single account? They sign one W-9 not multiple.

So they have to choose correct?

Correct.