We study the structure of resultants of two homogeneous partially composed polynomials. By two homogeneous partially composed polynomials we mean a pair of polynomials of which one does not have any given composition structure and the other one is obtained by composing a bivariate homogeneous polynomial with two bivariate homogeneous polynomials. The main contributions are two equivalent formulas, each representing the resultant of two partially composed polynomials as a certain iterated resultant of the component polynomials. Furthermore, in many cases, this iterated resultant can be computed with dramatically increased efficiency, as demonstrated by experiments. This paper is part of the author's work on resultants of composed polynomials.