Regulation of respiration part I (4-2013) by Dr Khaled A Abulfadle

To understand the answer, you should ask yourself

What is the effect of excess CO2 on respiration
What is the effect of oxygen lack on respiration

Excess arterial CO2 (within limits) increases respiration both directly (on peripheral chemoreceptors) and indirectly (on central chemoreceptors), but, Oxygen lack increases respiration ONLY through peripheral chemoreceptors (O2 lack receptors) in carotid and aortic bodies. Thus, both CO2 excess and O2 lack increase respiration and that is why their occurrence at same time augment their effect