Given the information you present, I'm going to say that your customer is the bank that is acting as agent for their trust customer. Under the preamble to the rule, "the final rule excludes from the definition of "customer" the following readily identifiable entities: a financial institution regulated by a Federal functional regulator; a bank regulated by a state bank regulator; ..."etc. So if the bank that is buying the CD on behalf of it's trust customer is state or federally regulated, they do not meet the definition of "customer" so the CIP rules do not apply.
Additionally, if the other trust is acting as trustee, they have a fiduciary duty to protect the interests of the beneficiary(ies). Trying to obtain a copy of the trust agreement from them would most likely not be successful, as that would fall under the sharing of information with non-affiliates. (BTW - The Trust Regulatory News that came out today contains an interesting article about fiduciary duties and sharing of information with affiliates and non-affiliates. It seens that the ABA and the legal scholars are taking different views. Interestingly enough the regulators and the DOL are remaining quite on the issue.)
Does anyone have anyother thoughts?