Could you interchange 10k BTU air conditioner with 2 5k BTU air conditioner and get the same result? Or is there a reason why buying one 10k BTU air conditioner is better than having 2 5k BTU air conditioners.
The 10k A/C will have a larger compressor, but only one fan, one control circuit, etc. Two 5k A/Cs will have two smaller compressors, two of everything.
I suspect that as a result, the 10k will be more efficient overall. What you should look at is the power consumption of each - then double the 5k. I expect that overall the 10k will run for less than 2 5ks - but I may be wrong. Depends on the power drain of the various components...
If one 5k unit will often be sufficient in your climate/room situation, it would be more efficient, as it will run longer cycles and lower the humidity better than the 10k unit (humidity is a key function of warm-weather comfort). If it's likely that both will be required often, then I would look at the specs; Greenbo is probably right in his analysis, but the specs would tell you for sure.