There's no specific prohibition on a lawyer representing Co-Trustees of a Trust. That being said, representing co-trustees can create ethical issues for the lawyer.
For example, if one of the trustees had prior dealings with the lawyer, or if one of the trustees shares confidential information that he or she does not want to share with the other trustees, or if a trustee needs to be shielded from liability from the actions taken by the other trustees, the lawyer may be placed in a situation where he is unable to represent all of the co-trustees fairly.
As a result, a lawyer must inform the co-trustees of the possibility of conflicts, and must also advise the trustees that should an actual conflict arise, the trustees will need to seek a new lawyer or a lawyer to deal with the actual conflict. Often, making the trustees aware of the lawyer's position at the outset helps to set the expectation that the co-trustees are to work in a cooperative manner to carry out the common goal of administering the trust effectively.