How to have a single common colorbar for two faceted framed (time-varying) plotly sf maps

Question

I have paired map display, with common frame slider, that currently looks as follows:

I would like instead for there to be a common colorbar for both sub-maps, going the full distance down the display. (Instead of, at present, two colorbars a and b, which both go down half the display height.)

Bonus kudos for a solution that allows the limits of the colorbars to be set too!

The code to reproduce the figure:

library(sf)
library(plotly)
library(tidyr)
library(dplyr)


fname <- system.file("shape/nc.shp", package="sf")
nc <- st_read(fname)

plot_ly(nc)

num_features <- dim(nc)[1]

# for each feature, imagine 10 years of data 

years <- 2000:2010

# for each feature, and for each of the 10 years, imagine there are two variables a and b

nc_names <- nc[,"NAME"] %>% st_drop_geometry()

set.seed(12)

fake_dta <- 
  expand_grid(nc_names, year = years) %>% 
    mutate(a = runif(n()),
           b = runif(n())
    ) %>%
  
  mutate(
    text_a = glue::glue("In {NAME} in {year}, a was {round(a, 2)}"),
    text_b = glue::glue("In {NAME} in {year}, b was {round(b, 2)}")
  )

# Join the data back 

nc2 <- nc %>% 
  left_join(fake_dta)
  

# Question is how to plot both a and b as two maps, side by side, sharing the same 
# time slider 

# to map a
map_a <- 
  plot_ly(nc2) %>% 
    add_sf(
      split = ~NAME,
      color = ~a,
      frame = ~year, 
      stroke = I("black"),
      text = ~text_a,
      hoveron = "fills", hoverinfo = "text",
      showlegend = FALSE,
      type = "scatter",
      mode = "lines"#,
    ) %>%
  add_annotations(
    text = "A",
    x = 0.5, y = 1,
    yref = "paper",
    xref = "paper",
    yanchor = "bottom",
    valign = "middle",
    align = "center",
    showarrow = FALSE,
    font = list(size = 15)
  ) %>%
  layout(
    showlegend = FALSE,
    shapes = list(
      type = "rect",
      x0 = 0,
      x1 = 1,
      xref = "paper",
      y0 = 0,
      y1 = 16,
      yanchor = 1,
      yref = "paper",
      ysizemode = "pixel",
      fillcolor = toRGB("gray80"),
      line = list(color = "transparent")
    )
  )
map_a

# to map b
map_b <- 
  plot_ly(nc2) %>% 
    add_sf(
      split = ~NAME,
      color = ~b,
      frame = ~year, 
      stroke = I("black"),
      text = ~text_b,
      hoveron = "fills", hoverinfo = "text",
      showlegend = FALSE,
      type = "scatter",
      mode = "lines"#,
    ) %>% 
  add_annotations(
    text = "B",
    x = 0.5, y = 1,
    yref = "paper",
    xref = "paper",
    yanchor = "bottom",
    valign = "middle",
    align = "center",
    showarrow = FALSE,
    font = list(size = 15)
  ) %>%
  layout(
    showlegend = FALSE,
    shapes = list(
      type = "rect",
      x0 = 0,
      x1 = 1,
      xref = "paper",
      y0 = 0,
      y1 = 16,
      yanchor = 1,
      yref = "paper",
      ysizemode = "pixel",
      fillcolor = toRGB("gray80"),
      line = list(color = "transparent")
    )
  )

map_b

# These produce warnings: line.color doesn't (yet) support data arrays
# Only one fillcolor per trace allowed

# and both together

subplot(map_a, map_b)