We use heavy‑nucleus effective field theory to compute radiative corrections to two-neutrino double‑\(\beta\) decay (\(2\nu\beta\beta\)). Our main result is the first derivation of a universal radiative‑correction factor for double‑weak decays -- the analogue of the Sirlin function in single-\(\beta\) decay -- independent of nuclear matrix elements and excitation energies. This “double‑weak Sirlin function” depends on the individual electron energies as well as their relative angle and differs significantly from the approximation obtained by summing two single-\(\beta\) decay Sirlin functions. In addition, we calculate the nuclear-structure-dependent component of the radiative corrections and find that they can still be neglected at current experimental sensitivities. On the other hand, the double‑weak Sirlin function induces distortions of the electron energies and angular spectra that are comparable in size to the leading nuclear‑structure correction parametrized by the ratio of nuclear matrix elements, \(\xi_{31}\). Our results indicate that extractions of nuclear‑structure information and tests of the Standard Model from high‑precision \(2\nu\beta\beta\) measurements must include double‑weak radiative corrections, implying that recent extractions of \(\xi_{31}\) should be revisited.