Here's a knot that can be used to tie together two runners of equal width. Since it's symmetrical, both sides are equally strong, and you don't have to worry about which side is weaker. It's simple to tie and will untie easily even after being loaded.

Step #1 The trick to forming this knot is in making the girth loop so that it looks exactly as pictured. First flatten your runner neatly and get all the kinks out. Then take one end (A) and pass it through the loop of the other end (B). Practice forming this girth loop until you can do it consistently. Make sure you don't tie the mirror image loop by mistake. This is the hard part.
Step #2 Now pass the second runner through the first girth loop and tie another girth loop EXACTLY the same way as the first runner. OK. You're done. Just tighten it.

Once you've tightened the knot, you will notice two distinctive bands running diagonally across the front and a spiral pattern on the back. You'll also notice that the runners enter and leave the knot at a 90 degree twist away from each other.
How strong is it?
This knot is WORSE than a simple girth hitch. I sent Chris Harmston two samples:

• Sample#1 Spectra/Spectra failed at 3421 lbf (15.2kN). This is only 56% of rated strength of 27kN.
• Sample#2 Spectra/Nylon failed at 1678 lbf (7.5kN). This is only 49% of tested strength of 14.9kN.