Database
[{id:"xyz",prop1:"val1",prop2:"val2",array1:[{arr1objid:"1"},{arr1objid:"2"},],array2:[{arr2objid:"a"},{arr2objid:"b"},]}]
Query
db.collection.aggregate([{$facet:{array1:[{$unwind:"$array1"},{$project:{array2:0}}],array2:[{$unwind:"$array2"},{$project:{array1:0}}],}},{$project:{roots:{"$concatArrays":["$array1","$array2"]}}},{$unwind:"$roots"},{"$replaceRoot":{"newRoot":"$roots"}}])