The heavy quark effective theory vastly reduces the weak-decay form factors of hadrons containing one heavy quark.Many works attempt to directly apply this theory to hadrons with multiple heavy quarks.In this paper,we examine this confusing application by the instantaneous Bethe-Salpeter method from a phenomenological per-spective,and give the numerical results for Bc decays to charmonium where the final states include 1S,1P,2S,and 2P.Our results indicate that the form factors parameterized by a single Isgur-Wise function deviate substantially from the full ones,especially when excited states are involved.The relativistic corrections(1/mQ corrections)re-quire the introduction of more non-perturbative universal functions,similar to the Isgur-Wise function,which are the overlapping integrals of the wave functions with the relative momentum between the quark and antiquark.