[{field1:[1,2,3,4,5],field2:["a","b","c","d","e"],}]

db.collection.aggregate([{"$addFields":{fields:{$reduce:{input:{$zip:{inputs:[{$map:{input:"$field1",as:"f1",in:{field1:"$$f1"}}},{$map:{input:"$field2",as:"f2",in:{field2:"$$f2"}}}]}},initialValue:[],in:{"$concatArrays":[[{"$mergeObjects":"$$this"}],"$$value"]}}}}}])