Well actually Joonas didn't come back for a while. During this time the slow merging of the pair continued, their feelings of pleasure becoming more and more intertwined until neither could tell who was doing what.
When Joonas did come back from wherever he had gone for that short while, he found Maria unconscious on the floor with Jill no-where to be found. Upon closer examination he realised that Jill had become Maria's pussy (the clue was that it was quivering independently).
Joonas woke Maria up.
"Looks like you couldn't fight it" he said smugly, "although I must admit, that merging worked better than I thought it would"
"Merging? Where's Jill?" Maria asked, then she remembered what happened, "Oh god! Jill is my...my..."
"Yep" he said, "Your pussy"
Maria looked down at her pussy, after a second it shook a bit and she felt pleasure course through her.
"Want to know what she thinks?" asked Joonas, making Jill able to speak to Maria.