For internet dates ONLY, the utility of double-headers is if you wish not to waste time getting catfished or disappointed (by incongruent photos), you simply double-up. If these are girls you have already met, it's pointless.
I have been double-header-ing almost every weekend the last few months. But you have to pick one, and let the other one go. Keep the dates apart from one another. They are not to converse. Give a reasonable excuse (and be polite) as to why you have to cut the date short to the one you let go to keep the door open for future communication. I usually go with "Hey, I'm sorry but I have to cut this date short. You are real cute, and I def would like to see you again."