Hemming double width on the loom is difficult because of the two layers. One of the reasons that we do the double width sample with Harrisville Shetland wool is because if fulls beautifully, negating the need for hemming. Just tie the fringed edges into knots after taking it off the loom and opening up the fabric.
If you’re making a project that is not double width but stays closed at the bottom, like a bag, you can hem as normal, because the layers are meant to be together at that point. Then to have the top open up nicely you would just not hemstitch the top end.