You can splice and weld tubes from two forks using an inner sleeve machined to fit the ID of the steering tube. The splice should be as low as possible so it will not interfere with the stem. The inner sleeve should be as long as possible so it keeps the steer tube straight during the welding process. Leaving a 1/4" between the two steer tubes to be filled by weld, and a couple 3/8" holes on both sides of the splice through the steer tubes to also be filled with welding (plug weld). Keep the heat down by letting it cool between welds to prevent distortion. I did this with a mig welder but I don't see why it couldn't be done with brazing as well.