I can fully understand why Hitchcocks will only sell the crankcase in matched sets.
That is because, the two halves are doweled together and then the final machining of the various bearing bores are machined at the same time.
This is the definition of a "matched set" in the machining world.
This assures the bores all line up when the two halves are assembled.
Unfortunately, the location of all of the machined bores, with respect to the location of the dowels is usually not held to a very tight tolerance.
This can cause the bores that were machined in the right half of set "A" to be displaced from the bores in the left half of set "B" when the two halves are joined together using the dowels to position them.
If the RE dealer and the engineers/sales people at Royal Enfield know what they are doing, you should receive a "matched set" of engine cases, not just one side.