You could make a coupler, and join two shafts together. Something like the joint on a two piece pool cue, or walking stick.
I have read of guys who have experimented with "take down" arrows that have made up such things.
You could screw two shafts together, end to end, by using an 8-32 threaded rod to join two point adapters together, though you would want to epoxy the point adapters into the shafts, and square the ends of the adapters with something like the arrow squaring device, to make certain the shafts stayed in line with each other.
Otherwise, you could have someone with a lathe turn you some inserts that fit the inside of your chosen shafting with a snug push fit, to join the shafts.
Should also work for using aluminum shafting for atlatl darts.