You can book these cabins online. There is a box to check that says "accessible cabins". They are no more expensive that any other cabin in the same category. Where they are, how many guests they hold depends on the ship. Generally they hold between 3 - 5 (depending on the category) and are available as insides, oceanviews, balconies and concierge.
On the Magic and the Wonder, the balconies are mostly at the aft of the ship and it should be no issue to get 2 next to each other on deck 7 (as long as they are not already booked), they are not connecting.
On the Dream and Fantasy, for balconies, you could get two together but they will each connect to a non-accessible stateroom (not to each other), they are located mid ship. The Dream and Fantasy also have many oceanview cabins that are next to each other and these are located forward.
On the official
Disney cruise site, there are documents available which tell you what each of the accessible cabins are (how many they hold, what's in the cabin, the accessible features of each).