Can you make a legal custody agreement without going to court in Maryland?

As a Baltimore, MD child custody lawyer, I am qualified to answer your question.  Agreements with respect to child custody are unenforceable until they are made part of a court order.  The Court is not required to approve the custody agreement, even if it is part of a separation agreement prepared by lawyers.  This is because the best interest of the child is of such paramount concern, that only a court may make a determination that is enforceable.  Even after the Court issues an order, a change in circumstances with allow a part to seek a modification of custody.  Having said that, it is most often the case that a court will simply approve the agreement. In sum, you may enter into an agreement with the other parents regarding custody, however it is unenforceable until formally approved by a court.