[{_id:1,quantity:[2,1,3],reagents:[{name:"FirstItem"},{name:"SecondItem"},{name:"ThirdItem"}]}]

db.collection.aggregate([{$addFields:{reagents:{$map:{input:{$range:[0,{$size:"$reagents"}]},in:{name:{$arrayElemAt:["$reagents.name","$$this"]},quantity:{$arrayElemAt:["$quantity","$$this"]},}}}}}])