Complex sentence: David Childs, one of Silverstein's favorite architects, joined the project at the urging of Silverstein and developed a proposal for 1 WTC, initially in collaboration with Daniel Libeskind.
Split into simpler sentences: David Childs, one of Silverstein's favorite architects, joined the project after Silverstein urged him to do so. He developed a design proposal for 1 WTC, initially collaborating with Daniel Libeskind.