Here's why:
* Indirect Objects (IO) receive the direct object (DO). The IO is the person or thing that benefits from the action of the verb.
* Direct Objects (DO) are the thing that is acted upon.
For example:
* "I gave *her* (IO) a *gift* (DO)." "Her" receives the gift (DO).
If you try to include two things that would be indirect objects, you'll either end up with:
* Two direct objects: "I gave *her* (DO) and *him* (DO) a *gift* (DO)." This is grammatically incorrect.
* An indirect object and a prepositional phrase: "I gave *her* (IO) a *gift* (DO) *for him* (PP)." This is correct, but the second element is a prepositional phrase, not another indirect object.
Remember: A sentence can have one indirect object and one direct object, but it can't have two indirect objects.