To overlay density plots, you can do the following:
In base R graphics, you can use the lines() function. But make sure the limits of the first plot are suitable to plot the second one.
In ggplot2, you can do the following:
dat <- data.frame(dens = c(rnorm(100), rnorm(100, 10, 5))
, lines = rep(c("a", "b"), each = 100))
ggplot(dat, aes(x = dens, fill = lines)) + geom_density(alpha = 0.5)