15 gallon is too small for even one O. scyllarus. Most people recommend 30 gallon minimum for a peacock. You could put a divider in a much larger tank, maybe 100 gallons, and keep one on each side, but I wouldn't put two mantis in the same tank space. Check out Dr. Roy's pages for specifics:
http://www.ucmp.berkeley.edu/arthrop...raca/royslist/