。
Theory and practice don't always go together.
Literal
Theory [and-と] practice [subject-が] accompany [quotative-とは] not-limited.
理論と実際が — using が rather than は puts the spotlight on the pair as the grammatical subject, subtly making the assertion about whether-or-not they accompany each other. 相伴う (to accompany one another) uses the reciprocal prefix 相. ~とは限らない softens to 'not necessarily.'