From my own experience, I charge my hourly rate plus the other sp's hourly rate. It is our time you are reserving, once in a while instead of doing 600hr we might drop to 500hr for a special or for a vip who is low-key like yourself. In most instances you reserve each sp's time individually instead of getting a better rate for each lady by booking both together. It's at least the same amount of effort in a duo, if not moreso... Interacting with 2 people not just one, and making sure there is a consistently sexy flow etc.
In any case, I always tell the ladies I play and work with that we, the sp's in the session should always be paid fairly and evenly, unless one is not involved as much, more as a bystander, or videographer, then another rate can be decided... If both sp's are kissing, bjs and doing full services and daty etc etc, both should be paid their full rate for their time, both are fully interacting, both have to get ready for the date, travel time, activities candles music etc etc... It's a package deal, and if you want the experience, it may be worth it! It may not be the type of indulgence a person would go for every time they want a play date, but you do say you enjoy it often, so perhaps look at your hobby and see if you are overindulging perhaps, we don't intend to bankrupt anybody or be greedy or anything like that. It's a treat a person would savour once in a while, and something to save up for if that's what you really want to do, much like a bucket list item.
Some ladies do ask for extra for another person to join. Whether it's your SO or another sp or gf it makes no difference. It's more intense to bring in a third person, that's also why some sp's only session with certain sp's, and prefer it that way. It is a lot based on chemistry and comfort.
P.S. Loved the group session we had a while back, island1-1, that was very spicy indeed! xoxox