Can I use two 20' 2" square-edged boards (one above the other) as a vertical fascia on a deck? What gapping would be appropriate between the boards?

Hello there, Thank you for your inquiry. We have not tested this type of installation with a 2" board, as it is too heavy. We would recommend to use Trex fascia which is manufactured in 1x8x12 and 1x12x12 or Trex decking which is 1x6. The boards should be gapped away from the house or solid object by 1/4". The gapping between the boards should be 1/8" end-to-end and miter cuts should be 1/8". If you decide to install deck boards, and you use the recommended fasteners according to our installation guide, you would want to install 3 screws every 12" and you would also use a weather-resistant, construction-grade adhesive (that works well with wood) as a secondary fastener. We have our installation guide on Trex.com under Literature. I hope this information is helpful to you. Thank you, Jessica -Trex Customer Care Team