Partial support of a beam
When your goal is to prevent rot, the best thing is to put those beams tightly together and then seal it all up with construction adhesive, or put a flashing on top of the beam to prevent water getting between the boards.

Having a space between the boards will allow tree debris to accumulate in there, and you basically have two separate beams (since they cannot as easily 'share' a load or transfer forces between them).