// from PA1
var lastSnapshot =tw.system.model.findProcessAppByName("PA2").snapshots.length-1
tw.system.model.findProcessAppByName("PA2").snapshots[lastSnapshot].findProcessByName("BPD2").startNew(new tw.object.Map(), false))
// If you know snapshot name of PA2 you can do
tw.system.model.findProcessAppByName("PA2").findSnapshotByName("snapShotName").findProcessByName("BPD2").startNew(new tw.object.Map(), false)
If BPD2 has input parameters you need to set them before starting the process
var inputs = new tw.object.Map();
inputs.put("paramName1","paramValue1")
inputs.put("paramName2","paramValue2")
tw.system.model.findProcessAppByName("PA2").findSnapshotByName("snapShotName").findProcessByName("BPD2").startNew(inputs, false)