如何设置哪个数据源将受到表单上下一个和上一个的影响？

I have a form with two datasources (DS1,DS2). I set the datasource property on design to DS1. Until some trigger happen the previous and next are calling methods of DS1. Then I enable FormStringControl with datasource set to DS2 and the previous and next start calling methods of DS2. I need to ensure the metods will be called on DS1 everytime. I have a form with two datasources (DS1,DS2). I