Adding/Merging node to boost property tree

I am trying to merge two boost property trees using put_child. But I am seeing the output as follows

<roottag>
    <city>
        <town>lmn</town>
        <town>rst</town>
        <town>def</town>
    </city>
    <parking>1</parking>
    <address/>
    <total>1500</total>
    <Customer1>
        <Car>
            sampleCar
            <daysCar>10</daysCar>
            <numberCar>1234</numberCar>
        </Car>
        <Bike>
            testBike
            <daysBike>5</daysBike>
            <numberBike>4567</numberBike>
        </Bike>
    </Customer1>
    <Customer2>Sample2</Customer2>
</roottag>

The two independent trees look like these

First tree:

<roottag>
    <city1 type=","/>
    <city type="|"/>
    <parking>1</parking>
    <address/>
    <total>1500</total>
    <Customer1>
        <Car>
            sampleCar
            <daysCar>10</daysCar>
            <numberCar>1234</numberCar>
        </Car>
        <Bike>
            testBike
            <daysBike>5</daysBike>
            <numberBike>4567</numberBike>
        </Bike>
    </Customer1>
    <Customer2>Sample2</Customer2>
</roottag>

Second tree:

<roottag>
    <city1>
        <town>abc</town>
        <town>xyz</town>
        <town>pqrs</town>
    </city1>
    <city>
        <town>lmn</town>
        <town>rst</town>
        <town>def</town>
    </city>
</roottag>

How to get both city and city1 added to the first property tree?