Individually, these two are totally fine JSON strings.
[{"`19 January 2021`":[["36","817","67"]]}]
[{"`20 January 2021`":[["79"]]}]
You can't parse them both together at the same time just by concatenating them though. Either parse them individually:
JSON.parse(arr1String);
JSON.parse(arr2String);
Or combine them into a single JSON object.
echo json_encode([
"arr1" => $arr1,
"arr2" => $arr2
]);
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