The theoretical foundations of the double hybrid exchange-correlation functionals have been recently analyzed by Sharkas et al.,1 and, successively, by Brémond and Adamo2 and by Toulouse et al.3 This analysis partially resulted in the introduction of a new class of double hybrids depending on just one parameter, the value of which was assumed to be 0.5 by Brémond and Adamo.2 In this note, I will suggest that other values can be chosen and that all these choices can be justified using the same theoretical arguments. These values are also "theoretical" mixing coefficients for single-hybrids, i.e., functional where only the exchange is hybridized.