merge.xml

<Map srs="+proj=utm +zone=32 +ellps=WGS84 +datum=WGS84 +units=m +no_defs">

  <Style name="color relief style">
    <Rule>
      <RasterSymbolizer mode="normal" />
    </Rule>
  </Style>  
  <Style name="hillshade style">
    <Rule>
      <RasterSymbolizer opacity="0.6" mode="multiply" />
    </Rule>
  </Style>

  <Layer name="color relief">
    <StyleName>color relief style</StyleName>
    <Datasource>
      <Parameter name="type">gdal</Parameter>
      <Parameter name="file">2012250.tif</Parameter>
    </Datasource>
  </Layer>
  <Layer name="hillshade">
    <StyleName>hillshade style</StyleName>
    <Datasource>
      <Parameter name="type">gdal</Parameter>
      <Parameter name="file">nigeria.tif</Parameter>
    </Datasource>
  </Layer>  

</Map>