Malibu is correct. I JUST finished this with exactly the same materials, for the same setup (one #6 feeder for two welding outlets).
Home Depot has everything you need. Get the COPPER split bolt connectors (forgot the size, but 3 quantity #6 wires was no problem). They have a variety of sizes, they are typically near the zip ties in with the electrical supplies, in plastic hanging bags. You will need 3 of these.
You will also need the padded double-sided electrical tape (the really thick stuff). Forgot the number/designation for this, but it's the only double-sided stuff I saw.
Last but not least, you will NOT be able to do all this in a 4" box easily (not even the deep ones). I bought a larger 6" X 6" box that works very well.
Process:
1) get all three wires into the box and strip appropriately (about 1/2")
2) get split bolts ready (have appropriate wrenches close).
3) insert wires into split for best fit. On one I did them all from one side so the wires were bundled. On another, I needed a flatter configuration so I did the top from the left, the middle from the right, and the bottom from the left as well. Go with what works best for your situation.
4) Wrap entire exposed mass in the two-sided electrical tape, this creates a cushion and insulates the split itself.
5) The two sided stuff is sticky, so finish it all off with a coating of normal electrical tape.
