How about a competition to earn a breeding?

"WHAT WOULD YOU DO IN SERVICE OF YOUR MASTER?"

"Anything!" They both jump to their feet excitedly. They enthusiastically hug each other. It was clear they enjoyed doing everything together.

Your cock stirs further watching their breasts smash together under the shower water.

"HOW ABOUT A COMPETITION?"

"Really?" They look at each other uncertainly.

"YES. HOW ABOUT THE WINNER GETS A BABY PUT INSIDE HER BELLY?"

They positively beam at the idea.

"What do you have in mind Master?"