If I understand your situation correctly, your goal is to lower the groundwater table under your house. You have determined that the sump pump vault must be deeper in order to maintain the groundwater table at a lower level.
My approach to this situation is as follows:
I think that you should cut some slots with a saw, in the side walls of a plastic sump vault, and then install the vault into the deeper hole. Backfill around the vault with pea gravel, and install a new sump pump, along with a high level alarm system.
If you desire to have a back-up pump already installed, and plumbed into the system, then I would encourage you to contact a Licensed Master Plumber. The choice of a “Duplex Pump System” involves a somewhat complex installation procedure. In my view, a duplex system would be recommended if the sump pump is operating for extended periods of time, and if it is critical that the ground water level be controlled at all times.
If the purpose of the sump pump is to control an occasional, seasonal rise in the groundwater level caused by storm events, then I would recommend a single pump system that you can install your self.
If you decide on a single pump system, select a high quality, pump model that will be easy to replace in the future when this pump will inevitably fail. I recommend Zoeller Pumps highly.
I think that you should also divert the downspout roof drain to flow away from the home, into a storm drainage system. JWA