If it's the Schiphol location you're staying at (which is possible, given the usage of the airport code), then they may hit you with the second person charge, because it appears that they have an additional for the second person. (On my test booking, the in-city location did not.)

You could always just say nothing about the second person, and hope that no one notices. Personally, though, I'd book the second person now and eat the extra cost (about 25 Euro or so). If they decide to charge you for the second person at the time of check-in, they could well charge you the going rates for rooms (which could be more).

Besides, the extra money spent means more points.

Mike