It’s been a while since this one came up, but the last time it did (Suggestion - Barbershop Template) Daniel made the point that what really ought to happen is that you write music on four staves and Dorico then condenses it down to two automatically.
In the past few years Condensing functionality has come to Dorico - and this is exactly how you’d notate e.g. two flutes on a single staff in the score, but with separate Flute 1 and Flute 2 part layouts - but (IMO) it’s not yet really suitable for vocal music - there are complications where lyrics are concerned - and it’s Pro-only functionality so doesn’t appear on the iPad.
If you wanted to put four voices across two staves manually, that’s easy enough, but where you need lyrics above and below each stave that’s currently difficult on iPad (though probably easier with a Bluetooth keyboard attached).