I recommend the AndyMark cluster gears and cluster shafts which are specifically designed to be used to put two gears on one shaft without spacers (be careful, if the parts don't come from the same transmission family, spacers or machining might be necessary to get them to fit on the cluster shaft).
http://www.andymark.com/Hex-Shaft-Gears-s/237.htm
http://www.andymark.com/Shafts-s/266.htm
the super shifter cluster shaft is named as such, but the tough box cluster shaft is simply called the Toughbox Small Hex Shaft.