Board Thread:Boruto Discussions/@comment-1512913-20190608172919/@comment-32685487-20190610232730

He needed to use the Karma to absorb chakra from the second Ten Tails in order to take on the appearance of her Partner, so there is a chance that he got that Karma from her partner and that the second Ten Tails IS her partner, therefor he could use the Karma to transform into her partner and use his power.