how to make this join for multiple columns?

join_coalesce.R

library(tidyverse)

(a <- data_frame(ID = 1:20,
                D1 = sample(0:1,size = 20,replace = T,prob = c(0.8,0.2)),
                D2 = sample(0:1,size = 20,replace = T,prob = c(0.7,0.3)))
)
(b <- data_frame(ID = 1:20,
                D1 = sample(0:1,size = 20,replace = T,prob = c(0.9,0.1)),
                D2 = sample(0:1,size = 20,replace = T,prob = c(0.9,0.1)))
)

full_join(a,b,by = "ID",suffix=c("_a","_b")) %>% 
  mutate(D1 = ifelse(D1_a + D1_b==2,1,D1_a + D1_b),
         D2 = ifelse(D2_a + D2_b==2,1,D2_a + D2_b)) %>% 
  select(ID,D1_a,D1_b,D1,D2_a,D2_b,D2) %>% 
  mutate_all(as.integer)