<shrug> I sort of learned how to open sewn bag seams like that when I was a kid, from opening animal feed supplement bags while grinding steer feed with Dad. Still don't have it completely down, and there's definitely a right way and a wrong way. Every seam has what I think of as a "clean" side with even straight stitching, and a "rough" side. I start with the clean side at my right hand, and with the end farthest away from me. From the rough side, start separating the 2 threads, and start gently pulling from the clean side. If I haven't gotten the threads seperated right, sometimes I'll have to pull the clean thread through to the rough side, and unloop it from the rough side thread.