Are there cases where a double horizontal stabilizer is useful?
The main aerodynamic purpose of the horizontal stab (or certain canards) is to provide longitudinal stability. 

If the rear wing with the 5th and 6th engine flies "up," like the main wing, then it will counteract the longitudinal stability of the horizontal stabilizer. If the rear wing flies down, like the h-stab, then it is just extraneous, since the h-stab can be made as large as necessary.

If all you want to do is to add extra engines, they can be mounted on the main wing, like in the eight-engine B-52 bomber:



or the six-engine Antonov An-225 cargo jet: