library(tidyverse)
overlap <- tibble(
col_A = c(1, 2, 3, NA, NA),
col_B = c(10, 20, 30, 40, 50)
)
overlap %>%
mutate(col_A = ifelse(is.na(col_A), col_B, col_A))
#> # A tibble: 5 × 2
#> col_A col_B
#> <dbl> <dbl>
#> 1 1 10
#> 2 2 20
#> 3 3 30
#> 4 40 40
#> 5 50 50