No you do not need an airstone, sadly the SMW has no room on the inside for an air stone :(
You need to go from one input (from the pump) of air to 3 controlled outlets. 2X SMW, 1X extra air bleed off, If the pump makes too much air.
I would get a 3 way gang valve or a "air control kit"
3 way gang valve: http://www.petsmart.com/product/inde...ductId=2752202
Air control kit: http://www.petsmart.com/product/inde...ductId=2752256
Personally I like the air control kit, gives you way more flexibility for the future& is easier to hide. If you add or remove stuff, change where and how it works.
Gang valves: Especially if you cant find room to hang a gang valve on the back of your eclipse you have to have it sitting on the table behind the tank & is kinda clunky. The air control kit allows you to have one line going into the tank and then splitting it from there.
Here is kind of an example of what I would do: