db={"one":[{obj:{b:"ABC",a:123}}],"two":[{arr:[{b:"ABC",a:123},{a:234,b:"BCD"}]},{arr:[{a:123,b:"BCD"},{a:234,b:"ABC"}]}]}

db.one.aggregate([{$lookup:{from:"two","let":{x:"$obj.a",y:"$obj.b"},pipeline:[{$match:{$expr:{$in:[{a:"$$x",b:"$$y"},"$arr"]}}}],as:"matchFromTwo"}}])