If you have a complaint, say it straight out. Your attitude is the very definition of two-facedness.
Literal
Complaint [subject-が] exist [if-なら] clearly say [imperative-よ]. Your that attitude [topic-は] two-faced-compliance itself [copula-だよ].
面従腹背 is a 四字熟語 meaning 'outward obedience, inward defiance' — complying on the surface while secretly opposing. そのもの means 'itself' or 'the very embodiment of.' はっきり言えよ uses the imperative + よ for a forceful command.