What kind of pins do you like to use? These ones are quilting pins, I like then because they are long, but I am constantly bending them beyond repair.
Instead of baste stitching one midriff on and then adding the other I just pinned all three layers together at once. If you are at all worried about doing this, then just follow the directions. But if you want to I would just suggest using a lot of pins so make sure your gathers stay in place and evenly distributed.