Here it is (with the partition named "date"):
select
COALESCE("input1"."date", "input2"."date") as "date",
...
from "input1"
full outer join "input2"
... and "input1"."date" = "input2"."date"
where
"input1"."date" = '$DKU_DST_date' or "input2"."date" = '$DKU_DST_date'