How to merge 2 objects from the same path?

Question

I have the following structure:

billing: {
   bill1: { 
     creationDate: '2016-01-01', 
     buyer: "person1", 
     seller:"person2"
   }
}
people : {
    person1 : {
        name : "Jane"
    },
    person2 : {
        name: "John"
    }
}

And I join the paths the following way:

var norm = new  Firebase.util.NormalizedCollection(firebaseRef.child("/billing").orderByChild("creationDate").startAt(startDate).endAt(endDate),
                [firebaseRef.child("/people"),"people1","billing.buyer"],
                [firebaseRef.child("/people"),"people2","billing.seller"])
                .select("billing.creationDate",{key:"people1.$value",alias:"buyerDetails"},{key:"people2.$value",alias:"sellerDetails"); 
return norm.ref();

When I try this code I get 'Duplicate path' error on console, apparently this is a limitation, can you confirm?