How do I know which part of my Tiffany necklace will be engraved?

I will possibly be purchasing my daughter a Tiffany %26amp; Co. necklace, and it has two heart charms, one bigger than the other. I want to get the bigger heart engraved, but it doesn't give me an option. Has anyone ever gone through this process before? Thanks!!!!|||See if you can order it by phone, then tell them you want the bigger heart charm engraved. Worked for me! :]