You will need to create two different xml files, one for prediction and one for standard error. You already have one for prediction, and the easiest way to make it other is to make a copy of the xml file and change one line:
<enum name="KrigingResultType">Prediction</enum>
...should change to:
<enum name="KrigingResultType">PredictionStandardError</enum>
Aside from that line, the files should be identical. In ModelBuilder, you'll need to make a second branch in your model to run Create Geostatistical Layer and GA Layer to Grid twice (once with each xml source).