T item V1
1 a      2
2 a      5
1 b      1
2 b      2

T item V1
1 a       2
2 a       6

df1 %>%
  left_join(df2, by = c("T", "item")) %>%
  mutate(
    V1 = coalesce(as.numeric(V1.y), as.numeric(V1.x))
  ) %>%
  select(-V1.x, V1.y)