Just giving one dataframe as argument for ggplot() and the other inside the second geom_point declaration should do the work:
ggplot(aes(x='x', y='y'), data=df1) + geom_point() +
geom_point(aes(x='x', y='y'), data=df2)
(I prefer using the column name notation, I think is more elegant, but this is just a personal preference)