A double selective series (a)ii and (b)ii of dI/dV spectra recorded on a Pb(111) sample non grounded when both STM Tip A and Tip B are in tunnelling conditions. For reference, the (a)i and (b)i spectra have been recorded on Tip A and Tip B with the sample grounded. The α and β parameters are obtained by fitting the (a)ii and (b)ii spectra to the (a)i and (b)i dI/dV spectra recorded independently on the Tip A and Tip B dedicated lock-in when the sample was grounded. This fitting leads to the series (a)iii and (b)iii also perfectly superposed on (a)iv and (b)iv. V_A-S  = (V_A − V_S) the voltage difference between Tip A and the sample holder floating (see Fig. 2a), V _B-S  = (V_S − V_B) between Tip B and the sample holder floating and V_A-B = (V_A − V_B) between Tip A and Tip B. V_A-GS and V_B-GS are the Tip A and Tip B bias voltage when the sample holder is grounded (GS).