It wont work like that and if it did it wouldn't be because the dock has more ram in it.. it would be because the handheld has all the memory and wont have access to it when it is in portable mode so save on power.
I highly doubt they would split the memory pool between the dock and the handheld because it would be a nightmare for them to tell developers to think about 2 different memory pools.. and it just doesnt make sense from a design perspective.
At best that dock just has an upscaler in it to assist in the smooth transition from handheld to TV output.
What if that extra 2gb is actually ON the tablet Motherboard too, but just gets disabled when undocked to save power?