Miscibility of polylactide (PLA) and polyhydroxybutyrate (PHB) is studied by the microsecond atomistic molecular dynamics (MD) simulations for the first time. The model and the simulation protocol were confirmed through comparison of the glass transition temperature (Tg) with experimental data. It was established that PLA and PHB are miscible on the basis of the Flory-Huggins theory. Analysis of the mobilities of PLA and PHB subchains revealed that the blends have two transitions to a glassy state at the length scale of a few Kuhn segments, which is in line with the predictions of the self-concentration model. At the same time at the larger length scale a single transition to a glassy state was observed, suggesting scale dependence of PLA and PHB miscibility. This scale dependence was confirmed through the evaluation of the interchain pair correlation functions.