Legal Question in Wills and Trusts in California

Not only is it not against the law, it is very common.

It is indeed very common to have Co-trustees of a trust, particularly where family members are involved. For example, parents often appoint two or more children as co-trustees to administer their trust after they are gone. In appointing co-trustees on needs to be careful, however, to be sure that they will be able to get along and work together as that is essential in the administration of a trust.

The people that administer a trust are actually called trustees, and it is not illegal to have more than one trustee or even successor trustees. Generally, the terms of the trust instrument control, and if the trust instrument specifies this, it is not illegal.