reasoning behind this?
It's there primarily to give the fuel more time to fully evaporate at high rpm before it goes past the intake valve(s). Even with large injection bodies, the air's truly honking down the hole when you're deep into the good part of the powerband, and the last thing you want is any liquid fuel being vaporized after the combustion event begins.
An injector located in the port of the head is so close that at high revs there may not be enough time for the fuel to vaporize properly. Not all engines have this issue, but if you find it, you need to either change to a fuel that vaporizes easier, heat the air and/or the fuel, or increase the contact time.
It takes time to vaporize fuel, and in this case more distance = more time. Not a lot more, but enough.
Another potential benefit is that you can have more precision at low revs by only using the first injector: using only one means that the low fuel flows required at low power settings are easier to control precisely.
Bringing the other one upstairs on line smoothly is just a different problem for the injection mapping to deal with. :nod