I went on Garage Door Nation website to look at the conversion chart from 1 to 2 torsion springs. For my 1 spring, size 2"/0.250ID/30.5" length, they recommended 2 of 2"/0.207ID/24" length. I got this kit from Amazon for $64 with $11 overnight shipping even though I could get free shipping through Prime over the weekend, but I couldn't wait for 3 more painful days. Installed it followed YouTube video. Worked better than my old one, a lot quieter. Check the video if you want to install one yourself. Professional installation costs hundreds of $$$:
A standard double garage door is 7 ft. high by 16 ft. wide. Standard single doors are 7 ft. high by 8 or 9 ft. wide. Because the doors are so large, few home centers and only some garage door stores keep many doors in stock, so expect to order one instead of buying it off the shelf. Garage doors are available in wood, fiberglass and steel. Steel doors, like ours, are light, maintenance-free, affordable, readily available, and have an insulating value as high as R-19.