I can't say if it's worth is as you will no doubt have to modify the sheetmetal on the shell as well. Also due to the larger wheel well with the extra axle, you might also have to mod the floorplan a bit as some things might currently be in the area where the second axle wheel well is planned to be.
As for dual vs. single...for comparision I can offer you this info:
We owned a 2003 19' Bambi with a single axle. Towed great, no problems.
We now own a 2004 Safari 25' with a dual axle. First, it tows about the same, however when in the coach with the stabilizer jacks down, to has less jounce to it than the Bambi did. Also, if I get a flat on one axle, I can back the coach up onto some wood with the good tire and change the flat tire without a jack...... now if I get two flats on the same side, then that benefit goes right out the window!
You can also still drive a dual axle Airstream in a sort of limp mode for a few miles if one of the tires goes bad, the other three pickup the slack, until you can get to a service center as soon as possible.
Honestly, if it were me, I'd most likely keep it a single axle and either sell it or get a dual axle coach unless you have the time, understaning, money and desire to really want to do it.