We simulate the control of the spin states in a two-electron double quantum dot when an external detuning potential is used for passing the system through an anticrossing. The hyperfine coupling of the electron spins with the surrounding nuclei causes the anticrossing of the spin states but also the decoherence of the spin states. We calculate numerically the singlet-triplet decoherence for different detuning values and find a good agreement with experimental measurement results of the same setup. We predict an interference effect due to the coupling of T0 and T+ states. ; Comment: 4 pages, 5 figures