Hi Hayashi,
As far as I know, if both items from the same vendor are accepted, in standard system will always create PO with these 2 items. There is no standard functionality to let purchase to choose which item he would like to create follow-on document within the same RFx response.
To achive this, you might need to add a customer field as a flag at item level of RFx Response, so that buyer could indicate which items he would like to create follow-on document. However, this would require also some customer coding to handle with it when you start to create follow-on PO. This is only my though of the possible option.
Regards,
Wendy