Definition - What does Co-Trustee mean?
A co-trustee is a party who manages a trust together with at least one other trustee. When a trust is managed by co-trustees, each co-trustee typically has equal privileges and powers. They also share the full responsibility of properly managing the trust together. If one co-trustee becomes ill, another one can fill in until he or she is healthy again.
Justipedia explains Co-Trustee
The reason why multiple trustees are often used to manage a trust is because trusts can sometimes involve assets worth incredible amounts of money. For example, a single trust could have assets worth over $100,000,000. Trusts can also have complicated instructions given by the grantor. This means that one single trustee alone could be overwhelmed if he or she did not get any help in managing the assets. Co-trustees can split up the work between both or all of them.