I’m sorry for a delay but I hurt my back a couple of days ago and sitting on a chair in front of a computer was unbearable. Fortunately it’s slowly getting better
I’m not sure what do you mean by: “We are fairly sure the house was built above grade and have flooded twice already in 2.5 years”… Is your house on a concrete slab or elevated on pillars and the entire surrounding area is flooding?
Since the sump pump is activated so often an underground water table must be pretty high. There might be an underground aquifer, nearby river / stream (above ground or underground), water well, as well as favorable topographic and geological conditions for the floods to occur.
Although your project might work, there’s a chance that the exterior drain, being 2 feet lower than the interior one, will be flooded all the time, which would make the second pump running constantly. Ideally you should have the secondary drain discharging naturally, without any mechanical pumps, into some lower area… but that’s probably not an option.
To make sure that the water from the interior drain is flowing into the lower (exterior) drain / culvert without first activating your existing (interior) sump pump you should probably make a connection at the lowest possible point of the interior sump pump pit (its base or just above the base in a side wall). That way the pump’s switch wouldn’t be activated, maybe something like on the picture below?