Scotch tape the film together at the edges in the film rebate (not in the image area). You should be able to do it just at one edge.

Sometimes I shoot 4 (almost) identical 6x17cm exposures on a roll to make small albumen prints in quantity, and I print them on one 8x10" sheet, and I just tape them together on one edge, and I print them in a split-back contact printing frame. If they come a bit out of alignment, I can usually adjust them with half the back clamped down and half open, with the taped edge under the clamped side.