For most instruments in the Notion for iOS app, you are able to write for 2 voices per staff. If you are writing for a grand staff, you can have 2 voices on the treble clef and 2 voices on the bass clef. To select a voice, you will want to go to the 4th pane from the left in the palette. Select the shaded note above a hollow note for voice 1 and the hollow note above the shaded note for voice 2. Choose your voice prior to entering your notation.
If you choose the drum set as an instrument, you will have the ability to compose for up to 4 voices. Once you tap the drum set staff so that the green bar is on it, you should then see the drum pads show up as a virtual instrument. To the left of the drum pads, you will see v1, v2, v3, v4. Choose your voice prior to entering your notation.