"You can buy one, then activate it, and then buy the second," Kwan said. "But you cannot be charged for two at a single time of purchase. You always need to have one unlimited activated before the card can store another block of unlimited time." Additionally, Kwan said the particular Reddit user who asked the question should check with their credit card company about the double charge.
It's probably worth noting, though, that at $112 for a monthly pass -- $116.50 starting later this month -- that'd be a pretty damn valuable card and would suck twice as much if you lost it. So other than saving some time, or perhaps saving a few bucks by getting an extra monthly pass before that fare hike (you'd have to activate it before March 29th, anyway), why would anyone want to do this?
Tony Merevick is Cities News Editor at Thrillist and is always worrying about possibly losing his fare card in the hustle every morning onto the train. Follow him on Twitter @tonymerevick.