ggh4x: Grouping Sub-Groups together in Faceted Plots with guide_nested_axis

I would like to create a bar plot in which the discrete values of the x axis would be grouped into subgroups (see code below), when I have already faceted on another variable. I have used ggh4x to get me close. But I'm not quite there:

set.seed(9) 
dfx <- data.frame(
    x = rep(LETTERS[1:2], 50),
    Group = rep(c("Group 1", "Group 2", "Group 3"), c(33, 33, 34)),
    value = rpois(100, 10),
    facet_var = sample(rep(c("A", "B", "C", "D"),c(25,25,25,25)))
)

ggplot(dfx, aes(paste0(x, "&", Group), value)) +
    geom_col() +
    facet_wrap( ~ facet_var, ncol=2) +
    guides(x = ggh4x::guide_axis_nested(delim = "&"))

But would like it to group my "Group" column together so that it looks more like what I see in this example:

df <- data.frame(
  x = LETTERS[1:16],
  group = rep(c("Group 1", "Group 2", "Group 3"), c(5, 3, 8)),
  value = rpois(16, 10)
)

ggplot(df, aes(paste0(x, "&", group), value)) +
  geom_col() +
  guides(x = ggh4x::guide_axis_nested(delim = "&"))

Anyone can help?

Use interaction instead of paste.

library(ggh4x)
#> Loading required package: ggplot2
set.seed(9) 
dfx <- data.frame(
  x = rep(LETTERS[1:2], 50),
  Group = rep(c("Group 1", "Group 2", "Group 3"), c(33, 33, 34)),
  value = rpois(100, 10),
  facet_var = sample(rep(c("A", "B", "C", "D"),c(25,25,25,25)))
)

ggplot(dfx, aes(interaction(x, Group, sep = "&"), value)) +
  geom_col() +
  facet_wrap( ~ facet_var, ncol=2) +
  guides(x = ggh4x::guide_axis_nested(delim = "&"))

^{Created on 2023-03-22 with reprex v2.0.2}