Joint ownership, though that's not fully possible!

Momo was currently on a computer in the manor's library, doing a bit of side studying on how certain things are made to use her Quirk better, when Izuku and Setsuna located her. After confirming they were alone, she asked, "Hello Master, what do you need?"

"Hey Momo, is there a way to share ownership of a genie lamp?" Izuku asked Momo, who blinked at the question.

"...well, the best you could do is pass it back and forth," Momo replied. "In which case, you'd obviously have to trust whoever you shared it with to give it back when it was their turn to return it..."

"...yeah, no, I can't be trusted with that, as much as I hate to admit it," Setsuna replied, getting a look from Izuku. "...what? I'm a shameless slut that likes being in charge, I'm self aware enough to admit it. I might get tempted to screw with you guys a little beyond some limits over time," she replied. "I'm better off not having that kind of power. It's much safer in your hands."

"...well, at least you admit that," Izuku sighed, rethinking his idea. "So it's not possible to create a duplicate lamp, with equal power?"

"No, a genie must follow some form of hierarchy, and only one Master or Mistress can be at the top at any given time," Momo replied. "It's meant to prevent the possibility of power struggles between equals. Granted, there are still power struggles in other scenarios, but it forces the "lesser" to be creative about it. That said, it is possible to create an object with some degree of the Lamp's granting of ownership over me. It just has to have lesser authority than the owner of the Lamp themselves has. To what degree is up to the Primary Master's discretion."

"...okay, makes sense," Setsuna nodded, before smiling at Izuku. "I appreciate the thought, Izu-kun... how about a ring that gives me a degree of ownership, but doesn't allow me to make any wishes you would?"

Izuku smiled back. "That works." He turned to Momo, and made the wish, though also specifying that only Setsuna would be able to use it, given the risks of theft with Setsuna's Quirk potentially allowing the ring to be taken from a detached hand, and Setsuna soon had a silver ring with an emerald encrusted in it on her finger.

"...nice," she smirked, looking it over. "I can at least have a little fun mischief that doesn't cross any lines with this..."