Logical effort and delay estimation
Intuitively Logical Effort is proportional to the amount of "work" needed to propagate a signal through a gate. One way this "work" is done, is by charging the gates of the mosfets to turn them on. So in the case of 2 inputs active, you are charging two gates and hence moving twice the amount of charge to achieve this, therefore Logical Effort increase.

When estimating delay you can do it for one path(from desired input to output) at a time, so unless you are splitting the path and applying it to the same gate(which is redundant and wasteful in terms of logical effort), you always use "effort of 1 input"