[{"_id":ObjectId("5da5aee33e791547bb418782"),"list1":[{"document":{"list2":[{"sub-document":{"timestamp":100}},{"sub-document":{"timestamp":105}},{"sub-document":{"timestamp":110}}]}},{"document":{"list2":[{"sub-document":{"timestamp":101}},{"sub-document":{"timestamp":104}},{"sub-document":{"timestamp":109}}]}}]}]

db.collection.aggregate([{$unwind:"$list1"},{$unwind:"$list1.document.list2"},{$sort:{"list1.document.list2.sub-document.timestamp":-1}},{$limit:2},{$replaceRoot:{newRoot:"$list1.document.list2"}},{$sort:{"sub-document.timestamp":1}}])