db={"currentBills":[{memberId:1,name:"Mark",status:"paid"},{memberId:2,name:"Steve",status:"unpaid"},{memberId:3,name:"William",status:"paid"}],"pastBills":[{memberId:5,name:"Joe",status:"paid"},{memberId:8,name:"Carlos",status:"paid"},{memberId:25,name:"ben",status:"unpaid"}]}

db.currentBills.aggregate([{$addFields:{coll:"currentBills"}},{$unionWith:{coll:"pastBills",pipeline:[{$addFields:{coll:"pastBills"}}]}},{$group:{_id:{coll:"$coll",status:"$status"},members:{$push:"$$ROOT"}}},{$group:{_id:"$_id.coll",coll:{$push:{k:"$_id.status",v:"$members"}}}},{$group:{_id:null,root:{$push:{k:"$_id",v:{$arrayToObject:"$coll"}}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$root"}}}])