When someone says "double-hammer launch" I take it to mean that two people will set and a third will jump or dodge into it.
One type of this kind of double-hammer launch is when two players set their hammers to launch in the same direction, then they crouch (usually) and face each other. The third person (the launchee) then jumps or dodges between the two hammers, hitting both simultaneously and getting propelled a great distance. This usually appears at the start of Desolate, but no bugger ever knows where to aim or where to stand, so they fuck it up.
This can also be done with both launchers crouching beside each other, pointing their hammers at the same place and almost touching, so that the launchee can dodge across the face of both. This is usually used when both launchers want to stay out of sight, or when there is an obstacle stopping them from facing each other as usual. You'll often see this one set inside the doors of Desertstorm, after the Scud Missiles have been taken. It will always fail too.
The third type of double-hammer launch is when one player sets a hammer-launch, just like a single launch, then a second person hammer-jumps into the launcher's hammer. The disadvantage is that the launchee loses health, but the advantage is there's no real team organisation needed - just hammer into it if you see it. If you're playing Std maps you'll see this one a LOT in Overlord, under the sniper tower. Try running forwards and hammer-jumping into the hammer of the launcher, and you might reach the Gun Cavern window.