For anyone who wants to add RAM to a TwinStore here is a pic
Inside Twinstore You can see the existing RAM just to the right of the big black heatsink. The heatsink is in the way and needs to be removed to add the second RAM board. Just remove the 4 screws with the springs on them (you can leave the fan on the heatsink) then twist and lift the heatsink off. The heatsink has thermal compound on it so it will stick to the processor, be gentle but firm and it will come off.
There is a CR2032 battery in there also (middle bottom of pic), if your TwinStore is over 2 years old might as will put in a fresh battery while you are in there. I have no idea what this battery is for.