Hello, Bridget!
Purchasing onboard gifts and amenities ahead of your vacation is a great way to enhance your Disney Cruise Line experience. As you have experienced, the online shop limits purchases to no more than two alcoholic items per stateroom. Since you have two staterooms, you should be able to make two separate purchases using your two unique reservation numbers. Assuming your party will all be seated at the same table for dinners, it should not matter which stateroom reservation is used for the wine package. But I recommend having the 6 Pack and the Beer Mug Savings Package orders associated with the room or rooms where the individuals using them will be staying so they are delivered to the right place. Prices are subject to change. Yes, some items may be purchased after you board as well. But you will usually find the best prices when purchasing in advance, especially for the wine packages.
If you should have any trouble placing your orders online, you may reach out to the Disney Cruise Line by phone. The amazing Cast Members should be able to help sort out pre-cruise purchases of gifts and amenities. If you would like to bring alcoholic beverages onboard in your carry-on luggage, I recommend reviewing the Disney Cruise Line alcohol policy.
I'm looking forward to my family's upcoming cruise aboard the Disney Treasure too. If it's spirits you happen to be interested in, don't miss your chance to visit the Haunted Mansion Parlor. (See what I did there?) They even have spirit-free options, including bottled soda "potions" and other ghoulish delights!
Have a magical cruise, Bridget!
~Tony