Wade Ford says:
"But if you're making a tarp does it really matter that the left side is 1/2" narrower than the right side?"
Probably not, but it will matter a lot in something with panels that need to fit together properly, as in a tent or pack.
The usual way to make a mock felled seam is not with the edges of the fabric alligned, but with uneven allowances like this:
That's half inch on the bottom, quarter on the top. Then you fold the longer allowance around the shorter one, and stitch. That gives two visible lines of stiching on the side with the added bulk, one on the other.
And third pass or not as fancy strikes you.
You will see, particularly in home sewing patterns, instructions telling you to sew the allowances even, and then trim one of them. That's a pain to do, but it's another option. I don't trust my self to cut the right allowance, either, nor to not cut too much or too little.
 that first picture also shows why this is not considered a lapped seam. the two pieces don't over lap, they're superimposed, like a normal plain seam, but with the offset.