A couple of more thoughts.

Don't mistake "green lumber" for kiln dried. Some is, some isn't. If
you decide to go with this option, it will proudly reflect in the price
that it is kiln dried as well as be stamped accordingly.

On other thing, when assembling your beam, make sure your nails
penetrate both pieces of 2x completely. If you are using pneumatic
tools, use 10d or 12d. If you are hand nailing, use 12d or 16d and
bend over the points. Enough nails can keep the shear stress on the
beam down to almost nothing, and are an important integral part of this
live load component.

Robert