How to plot two histograms together in R?

Consider the below data frames −

> glucose   fructose 
We need to combine these two data frames but before that we have to make a new
column in each of these data frames to create their identification
> glucose$sweetener   fructose$sweetener  sweeteners 
Now let’s create the histograms
> library(ggplot2)
> ggplot(sweeteners, aes(length, fill = sweetener)) + geom_density(alpha = 0.2)