I've cut lines in half before for light rods but I would never do it on a 5-wt. On my 2-wt I rarely cast more than 30-35 feet so having only 40-45 ft of line on the reel is perfect but I regularly cast farther than that on my 5 weight and don't like to cast backing. Gel-spun backing may be an alternative. It is much smaller in diameter than dacron but is also pretty pricey. A WF line should take up less space on the reel than a DT as well allowing for more backing. Whoever said you really don't need much backing is probably right, depending on your fishing. The best bet is to just sell one of your reels like you are trying to do and buy a larger reel.