I've done this before purely to have some flowers in the house before my own get going. I grow them on in the pot until the flowers finis. I do give a good soaking in water with some tomato food in it and then drain and display. repeat as necessary. When the flowers are spent, I carry on feeding and watering in a sheltered corner outside and then, when the leaves have gone over, I plant them in the ground where I want them to grow next year.
I don't think it would be helpful to split them and plant them now at the appropriate depth as you would be burying about half their leaves and, at worst, they might rot and, at best, you'd be burying half the food factory for next year's flowers.