The reason why we have two verses in both of which G’d is reported as speaking (separately) although no mention is made of Avram making any response in between, is that the first verse commencing with ויאמר was not a verbalised version of what G’d “said,” but describes what he thought. Such constructions are not unique, other examples occurring in Genesis 27,41 ויאמר עשו בלבו, “Esau said in his heart, etc.” It is reported as Esau having said something, else how could his mother have heard it? Solomon, in Kohelet 2,1 also uses the word אמרתי when describing something he thought rather than something he had actually said.