One of the most pleasurable treatments for a happy slave is "Double Domination".
This means that two female doms (Dominatrix) have their way with a male sub. The most pleasing thing for the slave here is that he is usually allowed to suffer twice as the two mistresses goad each other on to ever meaner and harsher punishments. Especially nasty is when the sub is so focused on one mistress that he neither suspects nor sees the devious punishment of the other coming. A wonderful game here is ballbusting from both sides, while the mistress in front distracts him with slaps and dominatrix kisses, the mistress behind him kicks him extremely painfully in the balls without warning. He usually doesn't even have time to enjoy the pain but immediately gets another kick from the other lady.
Examples on video for this beautiful treatment for real fetishists can be found here: Double Domination!