Yes... there is a selvedge on BOTh sides of a double width piece of fabric. When you make an "8 yard kilt", typically, you order 4 yards double width, cut it and splice it together.

The problem is with "A-Symmetrical tartans" (tartans that aren't mirror images of themselves). Most tartans have a 1234321 kind of color scheme. Some tartans are A-Symmetrical which means they have a 12341234 type color scheme. When they're asymmetrical, you can't join them like you described unless you HEM them.

BTW... in most swatch books provided by mills, they put an * beside the asymmetrical tartans to remind you to order single width.