Do two 5k BTU air conditioners produce the same cooling power as one 10k BTU unit?
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...