do you mean dual levers?
I've thought about adding them a couple of times but never bothered as I don't think i'll need them
there are two options for levers that can be grabbed when holding the flat part of the bar or on the curve of the bar.
these are the old style extension lever which bolts onto the normal lever.
these can make it uncomfortable to ride with your hand on the top of the brake lever (normal position)
recently bikes are being fitted with cross levers that bolt independently to the main lever but pulling on the same cable, these allow hands to be positioned where you choose.