If you pair the beams up, you should glue them with construction adhesive. Just nailing or screwing them together won't really distribute the load across both beams. Although; you're probably not planning on hanging or resting anything on just one beam and expecting the other beam to help with the load anyway.
A pair of 2x8 beams are going to be 3" wide (typically 1/4" is planed off each side of dimensioned lumber to give it the finished surface).
A 4x8 beam is going to be 3-1/2" wide. The extra half inch is going to add some strength to the 4x8 beam.
The 4x8 might be more prone to warping than a couple of 2x8's if it's a solid beam (but not if it's a laminated beam).
If this was for indoor use, you could sandwich a strip of plywood between the 2x8 beams and glue (not nail) all three pieces together to create a laminated beam that would be stronger and less prone to warping than the 4x8. But since you're building a deck, the sandwiched plywood would soak up water and be a problem.