Well, as my predecessors have said here, the Lucky Egg is given to the Pokemon battling, and gives it a 50% increase in exp. gained.
The Exp. Share, on the other hand, is given to a Pokemon that isn't battling, and gives 1/2 of the experience of the battler to the Pokemon in the party holding exp. share.
Now what I think that you may be talking about is switching out the first Pokemon in your party holding an Exp. Share with another from your party during battle, and in that case the first Pokemon would gain 75% of the experience of the battle.
Hope that that answers your question.