You can use ggarrange from the egg package to get align the plots:
library(tidyverse)
library(egg)
key <- c("High", "Medium", "Low")
value <- c(.51, .36, .13)
df1 <- data.frame (key, value)
p1=df1 %>%
ggplot () +
aes (x = key, y = value) +
geom_bar (stat = "identity") +
coord_flip()
key <- c("5+ times a week", "3-4 times a week", "2 or fewer times a week")
value <- c(.87, .04, .09)
df2 <- data.frame (key, value)
p2=df2 %>%
ggplot () +
aes (x = key, y = value) +
geom_bar (stat = "identity") +
coord_flip()
ggarrange(p1, p2, ncol=1)

plot_grid from the cowplot package is another option.
library(cowplot)
plot_grid(p1, p2, ncol=1, align="v")

The patchwork package is a newer option. I don't have much experience with it, but it looks more flexible than the other options.
#devtools::install_github("thomasp85/patchwork")
library(patchwork)
p1 + p2 + plot_layout(ncol = 1)
