The Eastern and Western Guides were carefully planned to avoid dividing states. The Rocky Mountains form a natural biological and climatic division of the continent, and basically any state or Province entirely east of the Rocky Mountains will be covered by the Eastern Guide, while states and provinces in, or west of, the Rockies will be covered by the Western Guide. (Also see Which states are covered by the Eastern and Western Guides).
The only exception to this is the state of Texas, which has such a broad east-west axis that the only practical way to divide these guides was to put the 'Trans-Pecos' Big Bend country in the western guide, and the rest of the state in the east.
There is a great deal of overlap in the two guides, with over 650 species in the Eastern Guide and over 700 in the Western. But simply counting species gives a false impression, since many eastern species that are rare in the west are treated only briefly in that guide, and vice versa.