FAQ: Combinations of alleles of the same gene are indicated by writing the mutant names one after the other, for example PastelIno. A capital is used to indicate the start of each mutants name.
But how do we write the name of a combination with epistatic genes?
Well here we have an easy answer: exactly the same as we write alleles of the same gene! The mutant (gene) names one after the other. Why? It are also two (different) alleles that are creating a (non wild type) phenotype.