To get the page to update the multiple records that you would like to update you will need to have a recordset that lists all of the records with the checkboxes next to them. This page will then post to the multiple update page that has the DataAssist Search server behavior on it. The recordset that you have on this page will be filtered by the DataAssist Search server behavior based on the records that you have selected. This way when you perform the multiple update you can use this DataAssist Search filtered recordset to ensure that only the records you selected are updated.
Currently you have the multiple update page and the DataAssist Search server behavior on two pages. They should be on the same page. The first page in this process should only be listing all of the records with the checkbox next to them. You would then check what records to update and click on the button to post the information to your multi update page. The DataAssist Search server behavior will filter only the selected records for the recordset on this page.
You can make the page that has all of the records showing first and add in the checkboxes and have this post to the next page in the set. Before you add the multiple update to this page just add the recordset and the DataAssist Search server behavior. Test it out by checking selected records then going to this page to see the values from the recordset. You should only ever see the records that you selected on the first page. When you have it working like this you will be ready to add in the multiple update server behavior.