Two variable x-axis for boxplot in R using ggplot2

Question

I am trying to create a boxplot chart with a doubled X-axis. Defining the x variable as an interaction of two variables produces a single box, and thus I defined a single categorical variable that contains all combinations. But then how can I define a doubled X-axis? annotate would require a location on the x-axis, which is categorical.

This is my original code:

g<- ggplot(datR, aes(Cat,mean.R.friend)) 
 
 g + geom_boxplot(varwidth=T, fill="blue") + 
  theme(axis.text.x = element_text(angle=65, vjust=0.6)) + 
  labs(title="Boxplots of mean relatedness with contacts", 
       subtitle="N=100, 10 genes, Basic",
       y="Mean contact relatedness",
       x="")

All variables are numeric but Cat, which is a categorical variable containing labels of the different categories.