In a building that size I think you'd be much more satisfied in the long run with two 18K units rather than a single 36K. There's not a big difference in the equipment price for either combination, but the air distribution will be far better with two units rather than one.
The SEER on most 36K standard minis is around 16 while the 18K units are around 19, so you get 15% to 20% greater efficiency with the smaller units. Higher efficiency 36K units are available, but the price premium is hundreds of dollars.
Using numbers from Pioneer's site as I'm most familiar with their units, two 18K units would give you a range of cooling from 9K to 48K btu with both units running. A single 36K gives you a range of 16K to 48K btu. Numbers are similar among various brands, but the wider operating range at the lower end is advantageous during conditions where you need some cooling but not a lot, as it keeps the unit running rather than cycling on and off. Similar capacities apply in heating mode as well, with the obvious qualification that the ability to provide heat falls off as outdoor temperatures drop.
In deciding on one unit or two, it's worthwhile to keep in mind that mini splits have a lot of similarities to window units from the standpoint of air distribution. Multiple units with the same aggregate capacity will always give more uniform temperatures throughout the space as compared to one large single unit.