ggplot2 multiple scales/legends per aesthetic, revisited

I managed to get a satisfactory result by combining grobs from two separately generated plots. I'm sure the solution can be generalized better to accommodate different grob indices ...

library(ggplot2)
library(grid)

pd = data.frame(
  letters = strsplit("AGTGACCGACTATCATAGTGACCCAGAATCATAGTGACCGAGTATGAT", "")[[1]],
  species = rep(c("Human", "Armadillo", "Porcupine"), each=16),
  x       = rep(1:16, 3),
  change  = c(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
              0,0,0,0,0,0,0,0,1,1,1,0,0,0,0,0,
              0,0,0,0,0,1,1,1,0,0,0,0,0,0,0,0),
  score1  = c(0,0,0,0,0,0,1,1,2,2,2,3,3,3,4,3,
              0,0,0,0,0,0,0,0,0,1,1,1,1,0,0,0,
              0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0),
  score2  = c(0,0,0,0,1,1,1,1,0,0,0,0,0,0,0,0,
              0,0,0,0,2,2,2,2,0,0,0,0,0,0,0,0,
              0,0,0,0,3,3,3,3,0,0,0,0,0,0,0,0)
)


p1=ggplot(pd[pd$score1 != 0,], aes(x=x, y=species)) +
  coord_fixed(ratio = 1.5, xlim=c(0.5,16.5), ylim=c(0.5, 3.5)) +
  geom_tile(aes(fill=score1)) +
  scale_fill_gradient2("Score 1", limits=c(0,4),low="#762A83", mid="white", high="#1B7837", guide=guide_colorbar(title.position="top")) +
  geom_text(data=pd, aes(label=letters, color=factor(change)), size=rel(5), family="mono") +
  scale_color_manual("Change", values=c("black", "#F2A11F"), labels=c("None", "Some"), guide=guide_legend(direction="vertical", title.position="top", override.aes=list(shape = "A"))) +
  theme(panel.background=element_rect(fill="white", colour="white"),
        axis.title = element_blank(),
        axis.ticks.y = element_blank(),
        axis.text.y = element_text(family="mono", size=rel(2)),
        axis.text.x = element_text(size=rel(0.7)),
        legend.text = element_text(size=rel(0.7)),
        legend.key.size = unit(0.7, "lines"),
        legend.position = "bottom", legend.box = "horizontal") +
  ggtitle("Voila, the Score2!")

p2=ggplot(pd[pd$score2 != 0,], aes(x=x, y=species)) +
  coord_fixed(ratio = 1.5, xlim=c(0.5,16.5), ylim=c(0.5, 3.5)) +
  geom_tile(aes(fill=score2)) +
  scale_fill_gradient2("Score 2", limits=c(0,3),low="#1B7837", mid="white", high="#762A83", guide=guide_colorbar(title.position="top")) +
  geom_text(data=pd, aes(label=letters, color=factor(change)), size=rel(5), family="mono") +
  scale_color_manual("Change", values=c("black", "#F2A11F"), labels=c("None", "Some"), guide=guide_legend(direction="vertical", title.position="top", override.aes=list(shape = "A"))) +
  theme(panel.background=element_rect(fill="white", colour="white"),
        axis.title = element_blank(),
        axis.ticks.y = element_blank(),
        axis.text.y = element_text(family="mono", size=rel(2)),
        axis.text.x = element_text(size=rel(0.7)),
        legend.text = element_text(size=rel(0.7)),
        legend.key.size = unit(0.7, "lines"),
        legend.position = "bottom", legend.box = "horizontal") +
  ggtitle("What about Score2?")


p1g=ggplotGrob(p1)
p2g=ggplotGrob(p2)

combo.grob = p1g

combo.grob$grobs[[8]] = cbind(p1g$grobs[[8]][,1:4], 
                              p2g$grobs[[8]][,3:5], 
                              size="first")

combo.grob$grobs[[4]] = reorderGrob(
                          addGrob(p1g$grobs[[4]], 
                                  getGrob(p2g$grobs[[4]], 
                                          "geom_rect.rect", 
                                          grep=TRUE)), 
                          c(1,2,5,3,4))
grid.newpage()
grid.draw(combo.grob) 

I would use size of text for indicating the score2:

ggplot(pd[pd$score1 != 0,], aes(x=x, y=species)) +
  coord_fixed(ratio = 1.5, xlim=c(0.5,16.5), ylim=c(0.5, 3.5)) +
  geom_tile(aes(fill=score1)) +
  scale_fill_gradient2("Score 1", limits=c(0,4),low="#762A83", mid="white", high="#1B7837", guide=guide_colorbar(title.position="top")) +
  geom_text(data=pd, aes(label=letters, size = score2, color=factor(change)), family="mono") +
  scale_size_continuous(range = c(4, 8)) +
  scale_color_manual("Change", values=c("black", "#F2A11F"), labels=c("None", "Some"), guide=guide_legend(direction="vertical", title.position="top", override.aes=list(shape = "A"))) +
  theme(panel.background=element_rect(fill="white", colour="white"),
        axis.title = element_blank(),
        axis.ticks.y = element_blank(),
        axis.text.y = element_text(family="mono", size=rel(2)),
        axis.text.x = element_text(size=rel(0.7)),
        legend.text = element_text(size=rel(0.7)),
        legend.key.size = unit(0.7, "lines"),
        legend.position = "bottom", legend.box = "horizontal") +
  ggtitle("What about Score2?")

UPDATED:

Here is a quick hack, I'm not sure if this is easy to inspect visually though...

library(ggplot2)
library(grid)
library(proto)

GeomTile2 <- proto(ggplot2:::GeomTile, {
  reparameterise <- function(., df, params) {
    df <- .$.super$reparameterise(df, params)
    if (params$ud == "u") 
      transform(df, ymin = y) 
    else 
        transform(df, ymax = (y-ymin)*0.8 + ymin, ymin = (y-ymin)*0.2 + ymin)
  }
  draw <- function(..., ud) {.$.super$draw(..., ud)}
})
geom_tile2 <- function (mapping = NULL, data = NULL, stat = "identity", position = "identity", ..., ud = "u") { 
  GeomTile2$new(mapping = mapping, data = data, stat = stat, position = position, ..., ud = ud)
}

ggplot(pd, aes(x=x, y=species)) +
  coord_fixed(ratio = 1.5, xlim=c(0.5,16.5), ylim=c(0.5, 3.5)) +
  geom_tile2(aes(fill=score1), ud = "u") +
  geom_tile2(aes(fill = score2), ud = "d") +
  scale_fill_gradient2("Score 1", limits=c(0,4),low="#762A83", mid="white", high="#1B7837", guide=guide_colorbar(title.position="top")) +
  geom_text(data=pd, aes(label=letters, color=factor(change)), size=rel(5), family="mono") +
  scale_color_manual("Change", values=c("black", "#F2A11F"), labels=c("None", "Some"), guide=guide_legend(direction="vertical", title.position="top", override.aes=list(shape = "A"))) +
  theme(panel.background=element_rect(fill="white", colour="white"),
        axis.title = element_blank(),
        axis.ticks.y = element_blank(),
        axis.text.y = element_text(family="mono", size=rel(2)),
        axis.text.x = element_text(size=rel(0.7)),
        legend.text = element_text(size=rel(0.7)),
        legend.key.size = unit(0.7, "lines"),
        legend.position = "bottom", legend.box = "horizontal") +
  ggtitle("What about Score2?")

Upper half indicates score1 while the lower for score2.