Two nonallelic genes produce a new phenotype when present together but fail to do so independently then it is called : -

Complementary genes are two non-allelic dominant genes that interact to produce a new phenotype only when both are present together; neither alone produces it.
Example: in sweet pea, purple flowers appear only when both C and P are present; CCpp and ccPP are both white.
Epistasis is when one gene masks the effect of another gene, and polygenes add up their effects (quantitative inheritance).
So such genes are called complementary genes.