That's because the Winter Masques rules aren't the only rules for dual-kith changelings. Rites of Spring has a section on the subject, proposing different prices for the merit: 2 dots for a same-seeming kith, and 3 dots for a different-seeming kith. The different-seeming option specifically explains that while the character gets the benefits of both kiths, they only get the blessing (and curse) of their own seeming. ...It also says that the merit is available at character creation only, in contradiction to Winter Masques, but you're the ST, so when it's available is your call in the end.

Information on the Dual Kith merit can be found on page 88 of Rites of Spring.