Is a single ductless HVAC system more efficient than two smaller ductless systems?
I can't say anything about the efficiency difference between single unit or minisplit systems for a single room, but for 809 sq ft, 24000 btu (2 tons of refrigeration) seems like a high estimate based on the information here:

http://www.energystar.gov/?c=roomac.pr_properly_sized

http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-cooling-loads

http://en.wikipedia.org/wiki/Ton_of_refrigeration

Assuming that your concrete block walls are uninsulated and allow air to move through them (true unless there's insulation and an air barrier you didn't mention), the high estimate is probably reasonable. You will save money and energy in the long run, however, if you insulate and seal the walls and ceiling with advice from an energy efficiency/home performance professional, then install an appropriately-sized system.