If you want to be totally "historically correct", of course, you'll want two lengths of single-width tartan sewn together. As a matter of convenience, though, you might want to just use double-width fabric and mention it verbally during your presentation. If you haven't seen it, you might also want to check out Matt Newsome's article on the possibility of the "great kilt" having been worn with loops and a drawstring: http://albanach.org/drawstring.htm

Length-wise. . .I have a 34-inch waist and have found a length of 4 & 1/2 to 5 yards of double-width tartan quite sufficient.