The "slip" as you refer to it is quite normal. Most people have never pedaled a front freewheel bike so it feels different. What you are feeling is the amount of a revolution that it takes the pawls in the freewheel to engage. Now that you have 2 freewheels, front and rear, it can take even more of a revolution from the crank before they both are engaged and driving the wheel.
I would be more concerned about the getting your chain alignment right so that your chains will not interfere with each other. There are some tips in the instructions on how to resolve this. Your chain could stand to lose a link or two but it isn't that bad.