I have a considerable amount of water enter my basement sump pump pit when
it rains heavily, and a husky submersible pump which can pump several
thousand gallons of water per hour if needed when the pit begins to fill.
The pump is powered by 110V current, backed up by an automatic emergency
(natural gas powered) generator, so I feel quite confident I will be able to
pump water under most conditions.

The concern I have is if the pump fails.

I want to install a second pump which will kick in if the water level rises
in the pit high enough to trigger it. My current thought would be to mount
it above the current pump, and perhaps share the same outlet / discharge
pipe. I'm not sure this is a correct or optimal arrangement, or if there is
some better way of hooking up a second / backup pump. An alternative would
be to fit 2 pumps at the same height into the pit, let both operate whenever
water level rises, and then assume that either or both of the pumps will be
working when I really need them.

The basement is finished with a lot of relatively expensive tools,
furniture, etc. so I want this to be done right. I welcome any advice or
opinions, and thank you in advance for your assistance.

Smarty