The answer to this excellent question can be found in Chapter Nine of this excellent novel. This is of course the pivotal scene when Catherine declares her love for Heathcliff but also tells Nelly that it would "degrade" her to marry him. She therefore chooses to marry Edgar Linton, even though her love for him is so very different from her love for Heathcliff. Note what she says to Nelly, which of course the hidden Heathcliff overhears:
I've no more business to marry Edgar Linton than I have to be in heaven; and if the wicked man in there had not brought Heathcliff so low, I shouldn't have thought of it. It would degrade me to marry Heathcliff now; so he shall never know how I love him: and that, not because he's handsome, Nelly, but because he's more myself than I am. Whatever our souls are made of, his and mine are the same; and Linton's is as different as a moonbeam from lightning, or frost from fire.
Here we have the two Catherine's explained. There is an elemental, intrinsic connection between Catherine and Heathcliff that cannot be ignored. However, whilst he is away, Catherine remains tranquil and seemingly contented with her marriage to Edgar. It is only when he returns from his mysterious absence that their strong connection reasserts itself and the true Catherine, the real Catherine who is so temptestuous reveals herself once again.