Reaction stoichiometry for calculation of Kp
Each time one molecule of salt is decomposed, it produced $\ce{2 NH3}$ for $\ce{1 CO2}$. The number of molecules $\ce{NH3}$ is always double from that of $\ce{CO2}$ in the gas phase. Now the partial pressure of a given compound is proportional to the number of molecules of this compound in the gas phase. As a consequence, the partial pressure of $\ce{NH3}$ is always twice as high as the partial pressure of $\ce{CO2}$