for (i in seq(1, 2)) {
  dplyr::select(mtcars, contains('m'))[[i+1]] <- ifelse(is.na(dplyr::select(mtcars, contains('m'))[[i+1]]),
                                                        dplyr::select(mtcars, contains('m'))[[i]],
                                                        dplyr::select(mtcars, contains('m'))[[i+1]])
}

library(dplyr)
for (i in seq(1, 2)) {
  select(mtcars, contains('m'))[[i+1]] <- ifelse(is.na(select(mtcars, contains('m'))[[i+1]]),
                                                       select(mtcars, contains('m'))[[i]],
                                                       select(mtcars, contains('m'))[[i+1]])
}