I have used the band clamps a number of times and they work great. It is best to have the ends of the pipes butted together and to retighten the clamp after a few days. After that forget about it. As for the two size clamps just have an inch or so overlap in the pipes and you are good to go. You should retighten those also. They make it very easy to install electric cutouts etc. Be sure you get the widest ones because the cheap short ones don't grab enough pipe.