If you are getting a pair, are you expecting to breed them? If they are too be companions, get one parrot first then after the bonding and training process is over, then think about getting another one. This will keep you from going overboard and getting into a situation that you, as the flock leader, have to take responsibility for.
A cage can NEVER be too big for one bird. It is recommended by any veterinarian "the bigger the better". However, one inch bar spacing is too big for Lovebirds. Plumhead Parakeets, Rosellas, Ringnecks and Sengegals will be fine, but Lovebirds (basically anything smaller then a Cockatiel) may get their head stuck in the bars if they are more then 1/2 inch.
This cage is perfect for two of the bigger species, as long as they are not meant to be pets. If they are to be companions, I would stay away from Senegals, Ringnecks, Mini Macaws and Lovebirds. They are jealous by nature, and two in the same cage when they are to be pets is asking for trouble. Even other birds like Macaws, Cockatoos and African Greys are risky in this manner. They may attack their cage mate because they will be BOTH fighting for your attention.
If you are going to purchase a pair to breed, look for unrelated individuals or a "proven pair". A pair being proven means they have successfully hatched the previous owner healthy babies. However, ask for references and proof. Only 1 in 5 pairs of parrots actually ARE proven by owners.
If you want to get individuals from two different aviaries, don't just throw them in a cage. They need time to get used to each other, and this means placing their cages side-by-side (and you need a second cage) to learn about their future cage mate. This means minimal interference from you- meaning you give them the bare-bones attention so they would rather be with each other. This way, you do not have two fighting birds and perhaps a large vet bill along the way. Even bonded birds fight, and could possibly majorly injure each other. Sometimes two birds just cannot get along, and you have to be prepared for this unhappy, but none to rare occurrence.
It is also a good idea to have a second cage on hand, anyway. If one of the birds gets sick, or one of the parents’ shows a feather destructive behavior on its mate, or if one of the parents becomes aggressive towards the babies, you need the extra cage to separate them. This will keep disease from spreading and proper time-off during non-breeding seasons when they need a rest.
These are just some guidelines to help you make the best decisions for you AND your bird(s).