The total (mainplane + horizontal tail) lift on the aircraft must act at the C of G, otherwise there would be an unbalanced couple with the weight.
I'd like to modify that to say that "the vertical lift of the mainplane and the vertical lift of the tail must act at the CoG". After all, the vertical aerodynamical force on the tailplane is not directly linked to the force in the aircraft z direction.
I'm almost scared to put myself in the middle of all of this but hey, this place is usually civilized.
Regards,
Fred