Stamenmap: is it possible to mix zoom levels for different maptypes?

Hello,

I have an issue where I'd like to have a higher zoom level for a terrain than for the labels. At lower zoom levels the terrain looks too blurry and at higher levels there's too many labels. As in I'd like to be able to do something like...

``` r
terrain <- get_stamenmap(bbox, zoom = 6, maptype = "terrain-background")
labels <- get_stamenmap(bbox, zoom = 4, maptype = "terrain-labels")

#### I know this won't work, because you can't add two ggplots, but then something like:

p <- ggmap(terrain) +
ggmap(labels)
```

Is this possible with existing functions - I can't seem to work it out - or some functionality that can be introduced?