The “support factor” of a joist or beam is related to the vertical height. A couple 2x4s side-by-side have little value as support compared to a 2×8. Some people say it is ok to use two 2x4s stacked on top of each other IF they are thoroughly bonded (glued) to each other. I’d say that works if it is done properly in a shop, but that it is very hard to do it onsite.
I don’t have ready access to wood loading charts but your 20′ 5×5 is most likely a nominal 6×6, and I can’t imagine it would work at all for a 19′ span. However, there may be intermediate supports?
Sorry but I don’t understand what you mean by it being supported on both sides.