SELECT array (a.DocNum) for a in ActvCustOrdr end as Doc,
array(b.DocNum) for b in ActvMiscInven end as Doc2
FROM `transaction-data` td
WHERE td.`$MdfdTmstmp` BETWEEN "2022-07-30T00:00:00" and "2022-07-30T23:59:59"
AND td.RteId = "20024"
AND td.`$Type`="DailyReport"
SELECT (SELECT RAW a.DocNum
FROM td.ActvCustOrdr AS a
UNINON ALL
SELECT RAW a1.DocNum
FROM td.ActvMiscInven AS a1
) AS Doc
FROM `transaction-data` td
WHERE td.`$MdfdTmstmp` BETWEEN "2022-07-30T00:00:00" and "2022-07-30T23:59:59"
AND td.RteId = "20024"
AND td.`$Type`="DailyReport";
OR
If you are sure both of them arrays
ARRAY_CONCAT(ARRAY a.DocNum FOR a IN ActvCustOrdr END, ARRAY a.DocNum FOR a IN ActvMiscInven END)
OR
ARRAY a.DocNum FOR a IN ARRAY_CONCAT(ActvCustOrdr, ActvMiscInven) END
Otherwise
ARRAY_CONCAT(IFMISSINGORNULL(ARRAY a.DocNum FOR a IN ActvCustOrdr END, []),
IFMISSINGORNULL(ARRAY a.DocNum FOR a IN ActvMiscInven END, [])
ARRAY a.DocNum FOR a IN ARRAY_CONCAT(IFMISSINGORNULL(ActvCustOrdr,[]), IFMISSINGORNULL(ActvMiscInven,[])) END