How would you answer the following question using double object pronouns?
-A quiénes les escribiste las cartas? (a ellos) - How come, "Yo se las escribí " is not correct? I must follow write the answer according to what's in the parentheses.
-¿A quién le ofrece frutas Roberto? (a su familia) - ???
The problem with using double pronouns-- both direct and indirect pronouns together-- is that unless you've got the context of an entire conversation to clarify who and what you're talking about, the pronouns can become unclear. For you English majors, this is called an ambiguous antecedent.
So in answering your example questions, you may need to specify to whom you're sending the letters or offering the fruit:
(Yo) se las escribí a ellos.
Roberto se las ofrece a su familia.
It may seem redundant, but it helps to make the message clear. I've seen literary works in Spanish that even take it one step further. As in:
A ellos se las escribí las cartas.
A su familia Roberto se las ofrece las frutas.
(This seems incredible redundant to me, but it may have some stylized usage that escapes me!)
What you have is correct. You may want to include "a ellos" after the phrase you have to make it clear what the "se" is referring to. For the next one, you will follow the same procedure.
Roberto ___ ___ ofreció a su familia.
Filling in the blanks is the only part that can be tricky.
Las escribí... ?