mongoose populate field without ref option

I have 3 schema:

var User1Schema = new Schema({
    name: {type:String , trim: true, required: true },
    status: {type:Number, required: true, default:1}
},{collection:"user_1"});

,

var User2Schema = new Schema({
    name: {type:String , trim: true, required: true },
    email: {type: String, required: true, index: {unique: true} },
    status: {type:Number, required: true, default:1}
},{collection:"user_2"});

and

var ConversationSchema = new Schema( {

    participants: [{user:{type: ObjectId, required: true}],
    creator: {type: ObjectId, required: true},

    status: { type:Number, required: true, default:1 }

}, { collection:"conversation" } );

In ConversationSchema I have creator field whic has now ref option, because it can be type of User1Schema or User2Schema.

How can I populate creator field using mongoose