Complex sentence: He suggests that biracial people belong in two worlds and none all at the same time and asserts that they experience uncertainty and ambiguity that can exacerbate problems identifying with their own racial groups and others (Gibbs, 1987).
Split into simpler sentences: He suggests that biracial people belong in two worlds and none all at the same time. That they experience uncertainty and ambiguity which can worsen problems people face identifying with their own racial groups and others (Gibbs, 1987).