This is just my limited experience, but whenever I've sewn two sheer panels together, I pin all around the edges in addition to pinning the seam area. That way, the top panel is less likely to slide off the bottom one. Also, I press the hems and casings before sewing with LOW heat. Then I take it nice and slow and use fairly long stitches. Hope this helps.

pinning close together and keeping an eye on the fabric for slippage is all you need. I would forget the tissue idea altogether.

You will have to launder those sheers eventually. You might consider laudering the fabric (use a pillowcase or lingerie bag) before you sew to make sure they don't shrink up on you and be too short afterwards.
I love sheer drapes, and have Croscill pinch pleat sheers that are slow but sure shrinking up on me over the years. I'm going to take the bottom hem down to get some length.

Before you start sewing, make test samples from scraps to get the right needle, stitch length, and type of seam (sheers will fray). You also might want to tape down a seam guide so you get straight stitching and seams.