what syterm are you using? Many double rail setups actually have cross bars supporting the frame over the tubes. I have a mix of WW machine works and NRS and the WWMW stuff nicely spans the tubes allowing the side rails to rest on either side of boat tubes. But an all NRS or DRE or other fitting based design will also put weight across the tube. WWMW and Cascade's arched (welded frame) truly rest on both sides of tube. There may be others but fitting frames all rely on some sort of cross support that puts at least a few cross bars bearing weight on the boat tubes.
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae