I need to build a footbridge across a creek and, due to soft and low land around it, will need a long span crossing the creek. I found this tutorial by Ron Hazelton where he makes a bridge with a 12' span with two beams supporting it, with supports only at the ends.
Each beam is made up of two 2x6 boards mated together with construction adhesive and bolts every 2'. This creates a single 4x6 beam.
Is there a reason for doing this with two 2x6x12 pieces instead of a single 4x6x12 board? Would it be as strong (or stronger) if I did the same with a 4x6x12?