I know that Phil Wood sells a 48 spoke tandem hub which is very reliable but quite expensive. I don't if any others are available.
"Quite expensive" is a gross understatement. Absurdly expensive is closer to the truth. Also, a Tandem hub has a dropout spacing of 140 mm or more. It won't fit most road, touring or MTB frames.
The need for 40 or 48 spokes for a single bike of any kind is a left-over from the era of plated spokes and weak rims. Modern 32 or 36 hole rims with DT or Wheelsmith spokes will hold up for years under any reasonable conditions. Current MTBs use 32 hole rims and 14 ga or 14/15/14ga spokes and they can be subject to stresses no touring bike will ever see. How about a 19" low gear being cranked up a 30% grade by a strong 190 pound rider? If those wheels survive, so will yours.
Finally 14/15/14 butted spokes are actually more durable than straight 14 ga spokes. The thinner center section is a bit more resiliant and cushions the spoke ends. Remember, except for severe mechanical damage, spokes always break at the head or the nipple end so the "weaker" center is not the failure point.
This message was edited by DaveB on 12-5-05 @ 10:32 AM