I took a deeper look into issues I was having using region splitting when specifying a region (but not a key for splitting).
The midkey calculation is off by one and when there are 2 rows, will pick the 0th one. This causes the firstkey to be the same as midkey and the split will fail. Removing the -1 causes it work correctly, as per the test I've added.
Looking into the code here is what goes on:
1. Split takes the largest storefile
2. It puts all the keys into a 2-dimensional array called blockKeys. Key i resides as blockKeys[i]
3. Getting the middle root-level index should yield the key in the middle of the storefile
4. In step 3, we see that there is a possible erroneous (-1) to adjust for the 0-offset indexing.
5. In a result with where there are only 2 blockKeys, this yields the 0th block key.
6. Unfortunately, this is the same block key that 'firstKey' will be.
7. This yields the result in HStore.java:1873 ("cannot split because midkey is the same as first or last row")
8. Removing the -1 solves the problem (in this case).