Sarepta Therapeutics (NASDAQ: SRPT) and Amicus Therapeutics (NASDAQ: FOLD) are two fast-growing biotech stocks that are trading at price-to-sales multiples north of 50, yet biotech investors may still want to add them to their portfolios despite their sky-high valuations. Both companies already market fast-growing, FDA-approved drugs and each is developing new drugs for rare diseases that could significantly expand their peak sales opportunity.