Adding multiple columns in a dplyr mutate call

This answer applies here as well; the following approach is both tidyverse-idiomatic and more performant than separate() (as of 2020):

set.seed(310366)
tst = data.frame(x=1:10,y=paste(sample(c("FOO","BAR","BAZ"),10,TRUE),".",sample(c("foo","bar","baz"),10,TRUE),sep=""))

library(dplyr)
library(purrr)

tst %>% 
  mutate(tmp_chunks = stringr::str_split(y, fixed("."),  n = 2)) %>%
  mutate(y1 = map_chr(tmp_chunks, 1),
         y2 = map_chr(tmp_chunks, 2)) %>%
  select(-tmp_chunks)

... Or if you don't want y anymore after splitting it, you can change the last line to

  select(-tmp_chunks, -y)

You can use separate() from tidyr in combination with dplyr:

tst %>% separate(y, c("y1", "y2"), sep = "\\.", remove=FALSE)

    x       y  y1  y2
1   1 BAR.baz BAR baz
2   2 FOO.foo FOO foo
3   3 BAZ.baz BAZ baz
4   4 BAZ.foo BAZ foo
5   5 BAZ.bar BAZ bar
6   6 FOO.baz FOO baz
7   7 BAR.bar BAR bar
8   8 BAZ.baz BAZ baz
9   9 FOO.bar FOO bar
10 10 BAR.foo BAR foo

Setting remove=TRUE will remove column y