The SCUBAPRO Super Cinch Strap is the most secure strap made. It only requires the one strap, but the small upper strap can be used to help with B.C. & tank height adjustment, as you mentioned. Also, as awap pointed out it does give additional security.
Take care in closing the strap. Use an open hand in closing the strap to avoid getting caught.
Make sure that you wet your NEW strap before putting it on the SCUBA cylinder.
After a few dives, you will need to tighten the strap due to normal stretching.
Dumpster Diver ( DD ), I'm not sure why your strap was difficult to adjust, it should be easy.
As far as height of B.C., I find that I position it a little higher, so that the top of the handle is even with the valve.
But, as DD pointed out, it is your B.C., fit it so it works for you!