For a pair you'd need a good amount of floorspace - apart from anything else, to fit the two of everything in! Something like a 40 gallon breeder tank is good - or a detolf. 650 square inches sounds good though. What cage is it? Tanks or bin cages tend to be best as it's hard to find cages with narrow bar spacing in the US. There is a very good one though - the Marchioro Kevin - it's quite expensive though - costs about $120.
The two of everything is so they can't get territorial over something and fight over it. So if they have one each they can both use one without fighting over it. Often they share one anyway, but having two means they have the option.
It's very common for pairs to end up fighting and having to be separated unless the cage is set up with two of everything and no levels and I believe it's also recommended to take them both out of the cage at the same time and not take one out and leave the other one in. All to avoid them falling out. But it is good for them to live in a pair as well, so if you're prepared then hopefully everything will go well and they will stay bonded
You might need to have access to a spare cage or bin cage handy, just in case they need separating. If you have a couple of bins you could use one as a temporary place while you turn the other one into a bin cage
Just in case.