The additional flow loss from 2 45's is based primarily in turbulence. When water flows straight througha pipe, this is considered laminar flow (smooth). When that water is forced to change directions, the flow becomes turbulent (and thus decreased slightly) over a certain length. With a 90, the flow returns to laminar in, let's say, 1.5". Well, with 2 45's, you have turbulence from one 45 that is not returned to laminar prior to the 2nd 45, thus the increased flow loss.