You can merge both legends by giving the same name to both aesthetics, since you haven't provided a REPRoducible EXample (reprex) here is an example using the iris
built-in dataset.
library(ggplot2)
ggplot(iris, aes(Petal.Length, Petal.Width, color = Species, fill = Species)) +
geom_line() +
geom_ribbon(aes(ymin = Petal.Width -1, ymax = Petal.Width + 1),
alpha = .2) +
theme(legend.position = "bottom") +
labs(title = "Example",
color = "Species",
fill = "Species")
Created on 2019-09-26 by the reprex package (v0.3.0)