The idea of a joint phenotype is an important one in human cultural evolution - where most traits are phenotypes of both particular human hosts and cultural creatures.
For example, when someone sings "happy birthday to you" the resulting performance is the produce of both host genes and song memes.
A recent paper by David Queller discusses the idea of joint phenotypes observing that most cases of conflict between organisms over how local regions of space should be organized can be phrased in terms of relative contributions to joint phenotypes.
With joint phenotypes it is often possible to quantify the contribution of each host by asking what proportion of the observed variation in the trait each is responsible for. Of course to do this, you have to be able to measure the trait in question, and often there are multiple ways of doing this that can potentially provide different results. For example, in the case of the "happy birthday to you" song, the average pitch of the notes in the song is likely be mostly the product of host genes, while the relative pitch of each consecutive pair of notes is mostly the product of song memes.
Nonetheless, the idea of measuring joint phenotypes in this way represents a powerful tool for students of cultural evolution seeking to quantify the relative influences of genes and memes on particular traits.