Water knot
Water knot is a rather well know way of joining 2 ropes when working with flat rope.
Begin with the two flat ropes you want to join together.
Tie an overhand knot in one. This will probably need some adjustment before all of it will lie ~flat~.
The other rope will now simply just trace the first rope. But backward. From working end to standing part.
Bring the second line into the knot, keeping it parallel and flush with the working end.
Bring it around, continuing to follow the first knot.
If you were tying the second knot without tracing, this is the part where you would have completed the loop.
Finish tracing. This last step is simply placing the end through the loop as if you were tying a normal overhand knot.
When tightened up carefully, you see that the knot forms a roughly pentagon shape Of the five sides, 2 are at top, 2 have lines coming out of them, and 1 is at the bottom.