... do working clearances require 15” from center of the two outside starters ...
I think you are over interpreting that 30" minimum width issue in thinking that EACH device must have 30", ergo 15" from center to either side. The rule is to avoid having someone cram stuff into a small closet space where a reasonable person cannot get into it to work on it, i.e. the shoulder width of that person standing in front of it to work on it. So if your overall width is 32", you are good.
BUT, as ptonsparky said, you MUST be able to open each door at least 90 degrees. If it is a combo starter with the breaker / switch handle on the door or there is something else on the door that hits a wall and prevents the door from opening 90 degrees, you are in violation.