A. 하고. Coordination in the sentence.
To get "The toy and the chair", in Korean the conjunction 하고 (and) would have to be attached to 장난감, the toy (1st object). In other words,
장난감하고 의자 - The toy and the chair
B. To make 의자, the chair be part of the subject, 의자하고 will have to be moved to the beginning of the sentence. This would give:
의자하고 장난감-[이] = The chair and the toy- [Subject marker] ->
의자하고 장난감이 책상 사이에 있어요 The chair and the toy are between the desks.