If the header spans more than 4 feet, it must be doubled and the loads transferred to double trimmer joists.

Do I need double joists?

Joist hangers are an integral part of a building frame. In some cases, it becomes necessary to place two or more joists side by side to add more vertical support or to support a large piece of horizontal lumber such as a rim joist. When this need arises, you will need double or triple joist hangers.

Does double joists increase span?

For example, if you double the thickness or number of joists in a floor, the distance the boards can span will increase by approximately 25%. But if you double the width of the boards, the distance the joists can span increases between 80% to 100%, even though you're using the same board feet of lumber.