I am also preparing to get a second bun in a few months!! 

Is Dean free roam, or does he have his cage door open and can go in and out freely? You can’t go wrong either way in my opinion, pen or cage. They’ll be able to interact through the bars during the pre-bonding / bonding process while they’re running about. I wouldn’t get one large cage just yet, because the bonding process could take a while depending. Sometimes you get love at first sight and other times it can take years. 

You definitely shouldn’t move them into one cage together straight away. They need separate homes for the first two months at least.

Generally, the bonding steps are as follows (after both bunnies neutered/spayed and given a month to recover)
– new bunny given two weeks to settle into its new home, not near the existing bunny
– after new bun is settled in, move the two bunnies near each other, but ensure that they can’t touch through the bars, both in the cage and during free roam time
– At the same time, start pre-bonding them for a month – so swap them between each others cages every other day so they get used to the smell of each other
– After a month of prebonding, if there’s no longer any aggression when they’re in the other’s cage (stomping, chewing on bars, lunging for the other rabbit through bars), you can introduce them in neutral territory
-Start with short bonding sessions, and work up to a full 48 hours together, at which point you can call them bonded.

The BUNNY INFO section has a useful overview on bonding

As S&L said, this website has very informational content on bonding. Just click on bunny info and there’s a whole article on it. There are also other articles that help you prepare for a rabbit.

It does look like you need to do a bit more research before you bring home the second rabbit. Two un-bonded rabbits should not stay in the same cage UNLESS they’re bonded. The cage that you currently have for Dean is fine for during the bonding session. Maybe get a second cage for the other rabbit while they’re bonding. But once they are bonded, I’d say get a pen. They sell them cheap on chewy.com. Just make sure it’s at least 36 inches high. With two bunnies, I’d get two pens so that they can have more space.

I personally have never bonded two rabbits but I am planning to do that sometime this year. Again, the bunny info section is extremely helpful for this topic. Also, on youtube there are some videos that are very helpful for bonding.

Good luck

I also forgot to add that even if your rabbits have a large enclosure, they still need out of cage playtime. Many rabbit owners say 4 hours minimum while others say 2 hours minimum. It mostly depends on your schedule but I still think that 4 hours is the best. I personally can’t do that most of the time since I have a very busy schedule, but I am trying to improve that. Once my schedule clears up, I will start giving my bunny more play time