2+2=4 not 2+2=Magikarp but in narutoverse 2+2=whatever kishimoto wants
Basically you have two watered down versions of the sage's chakra, by mixing those two you get a piece of the sage that awakens, it is his chairs that makes up the two, the vitality and physical life force embedded in the ashura portion may amplify the yin portion of the indra portion creating the rinnegan.