Where Every Scroll is a New Adventure
I am completely normal about migi and dali
Spoilers for the Migi and Dali anime watchers!
The double dolphin necklace may have began as a gift from Migi to his "first crush" which, after it broke, served the purpose of having a piece of each other with them to remember as the girl was moving towns, but now it is more sentimental and a parallel for the brothers I dare say.
The boys have been together since forever. They do everything together, they where there for each other, despite the many problems they faced (their mom's death, life before their adoption, navigating having friends and family and especially their fight when Migi learnt that Dali was the "girl" and so on) and now they go their separate ways. It is weird for both of them but they love and support each other and their decisions, keeping a dolphin necklace each, reminding them that, despite being away from the other, they are not forgotten and are always a "sacred" piece of the other.
And if you think about it, the necklace really is a depiction of the boys. In the beginning, both acted as one person, Hitori, (a double dolphin necklace) and gradually Migi and Dali felt comfortable with their own selves and developed their own relationships with their environment that they fully embrace their individuality (two dolphin necklaces).