I know the example below is not exactly correct because the clef change happens while the previous notes are still sustaining, but sometimes doing this is the best way to communicate the music.
Make sure you only have the upstem voice notes selected. To add the 8va, use the Shift+C popover, and close the popover with Alt/Opt+Enter instead of just Enter. that will make the 8va apply only to that voice.
This is similar to using Alt+Enter for (staff-)local time signatures, key signatures, etc.