For one, they both came from the same source so there is no rejection between them. Legion-7 also doesn't have a physical body as he is a spiritual entity so there is no clash in terms of physical location. Instead, Legion-5's slot for divine ability can be used as Legion-7's body to facilitate the fusion between them.
Inside Legion-5 are 9 empty bubbles. They are the slots for his divine ability. Legion-7 entered one of them and occupied it so a round translucent being with tentacles appeared in the slot. This led to the reduction of the burden on Legion-5.
Legion-5 decided to consider his future. "My first divine ability is decided. I have to get the dragon core for the second one. Let me make preparations for the others."
He thought to himself. "Which of these three divine abilities should I choose?"