Modify concrete stairs to two-step width.

Our front entrance is comprised of 10 concrete steps with a rise of 7" and a tread of 11.5" and a width of 113". I want to convert the lowest two steps to a wider platform style. Is there a standard tread size for taking two human steps? I was thinking of 2' tread so that a person would take two short steps, but I'm sure there must be a standard out there somewhere. I suspect that I'm a little short on my estimation.

Also wondering if it possible to add concrete to an existing set of concrete stairs without tearing out the existing. Our stairs contain a storage room accessible via the basement which we do not want to disrupt. The stairs were built in 1968, appear solid and have no discernible cracks or settling. Thanks.