Euro Pass YOuth Qs

You can buy Euro Pass Youth with your HKSAR, but not with your BNO. Because most people (incl. travel agents) cannot tell the difference between a full British passport and BNO, so to avoid any hassle, it's better to use your HKSAR passport.

They don't check where your visa is (well, at least when I brought it, they don't), so don't worry. Just remember which passport you use when asked to show your passport. Your passport no. will be written on the Youth Pass.