I'm trying to plot the boundary of different spatialPolygonsDataFrames in ggplot2 or in rasterVis gplot(), but I just can't find the way, they tend to overwrite the previous one; this is something quite common in mapping, having different symbols for each layer, it's something like this what I'm trying to achieve (done with base graphics)
library(raster)
mex <- raster::getData(country="MEX", level=1)
munis <- raster::getData(country="MEX", level=2)
plot(munis[munis$NAME_1=="Veracruz"|munis$NAME_1=="Oaxaca",], border="green")
plot(mex[mex$NAME_1=="Veracruz"|mex$NAME_1=="Oaxaca",], add=T, lty=1)
legend("topright", c("state boundary", "munic. boundary"), pch=14, col=c("black", "green"), bty="n")