One air conditioning unit or two is better?

A 700 sq. ft. requires a single window air conditioner of 14,000 to 16,000 BTUs, depending on factors like how many people are using the room, how sunny or shady it is, and if there are many electrical devices (computer, tv, fridge) in the space.

Like the others who have responded so far, I also recommend a single unit because:
1) Only one window taken up with the AC
2) You can't air conditioner part of an open space - it's all or nothing - so you'd have to have them both running all the time
3) The electrical usage will be greater for two small units is going to be greater than the electrical usage for one big one (compare the wattage on the units you're considering and do the math)
4) Two ACs means twice the waste when they inevitably need to be replaced - two compressors, twice the freon, twice the rubbish, so if the environment concerns you at all, that should make a difference.

Also, I recommend AGAINST choosing an AC larger than you need. An oversized AC will cycle on and off frequently, which is not only energy efficient, it means it won't dehumidify the air as well as it cools, so you'll end up damp and clammy indoors.

The size of the unit is more important than how many (we're talking AC units here....).

I always recommend getting an AC unit that is rated for more space than what you need. If an AC unit is to small for the space it will have to work overtime and not be very efficient.

In general, one large unit will be more effective than two smaller ones.

Given that your area would be contiguous, there's no reason for separate circulation.

For the open space, one is better. Two will require you use up two windows, which makes it difficult to open windows for outside air when the temperatures are cool outside. Two units will use more energy, especially if one is on low speed. A better option is to find an energy saving unit that turns off the fans when the unit reaches the set temperature.