I know i have another part of me.. But i don`t know where is he/she now?

However, i have promised to my mom looking for him/her as long as i was able..

But, how do i find him or she?

I just have the photo of this necklace

My grandma said that, the key shape necklace saved by her, while the heart-shape padlock for my twin....

God, i don't have an idea now, where i should finding him/her?