You will want to use the beaming tool in Notion for iOS to accomplish this. The beaming tool can be found in the 7th pane from the right in the palette. If you are working with an instrument that has a techniques pane, it will be the 8th pane from the right in the palette. The tool looks like two 8th notes beamed together with the beam extending off to the right. With this tool, just tap your score where you would like to separate a beam, and then tap the beaming tool. If you look at page 26 of the User Guide (accessed by tapping the icon of the ?), the beaming tool is item number 17 on that page.