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Theory and practice don't always go hand in hand.
Literal
Theory [and-と] practice [topic-は] always compatible [quotative-とは] not-limited.
両立する (to be compatible / to coexist) is built from 両 (both) + 立 (stand) — two things both standing together. ~とは限らない is the partial-negation pattern 'not necessarily'; 常に (always) adds the universal scope it qualifies. A measured academic statement.