You should not insert a conjunction after the second sentence because the instruction states that there are two sentences combined. Therefore, the end of the second sentence is the end of the idea. There are no elements for the conjunction to relate. If it was added, the cojunction would be left there hanging and the sentence would be grammatically incorrect (see 1 and 2).
1) Yesterday it rained all day long; therefore, I stayed at home
2) Yesterday it rained all day long, I stayed at home therefore*
