When begin to edit a row, call the 'getEditor' method to get these two datebox editor and bind an events on the first one, listen to some conditions and set the value for the second one. Please refer to the code below:
$('#dg').datagrid({
onBeginEdit: function(index,row){
var db1 = $(this).datagrid('getEditor', {index:index,field:'datebox1'});
var db2 = $(this).datagrid('getEditor', {index:index,field:'datebox2'});
$(db1.target).datebox({
onChange: function(value){
$(db2.target).datebox('setValue', ...)
}
})
}
})