High temperature annealing experiments of n-type double-doped (co-doped) GaAs:Te,Ge single crystal samples close to or above the equilibrium doping limit are presented and compared to annealing results of very highly doped GaAs:Te known for a long time, but still not clarified satisfactorily. An addition of Ge impurity to GaAs:Te shifted the equilibrium doping limit to a lower free electron concentration - a result which is difficult to describe within models of doping limit assuming an electrical compensation mainly by native acceptors. Hall effect under hydrostatic pressure up to 1.5 GPa allowed to directly measure the change of concentration of Ge-Ga donors caused by annealing, in addition to free electron concentration changes in GaAs:Te, Ge. GeGa donors were detected by the capture of free electrons by Ge DX states at high pressure, following the method described by Baj et al. [Phys. Rev. Lett. 71, 3529 (1993)]. In highly doped GaAs:Te, Ge, the measured ratio of changes of free electrons and Ge-Ga donors concentrations caused by high temperature annealing at 1100 degrees C, Delta n/Delta[Ge-Ga]= 4.2+/-0.6, supports the model of chemically bonded impurity-impurity molecules, probably of type Ge-Te-m (where m = 3 divided by 4 atoms), which may be consistent with recently proposed models of paired impurities: DDX or double-DX centers.