estaban complementando
-they/you were complementing
Imperfecto progresivo para el sujetoellos/ellas/ustedesdel verbocomplementar.

complementar

Diversas formas de cooperación Sur-Sur estaban complementando y aumentando la cooperación Norte-Sur.
South-South cooperation, in various forms, was now complementing and enhancing North-South cooperation.
Palabra del día
el guion