[paranoidrobot]

> they could always use the other capsule

This only works when you're talking about switching between the same system.

Astronauts who flew up on Starliner can't just get into a Soyuz and fly home.

Their space suits won't work, and they can't just borrow the other guy's - they're custom fit.

[cameldrv]

The space suits are custom fit, but if two astronauts are about the same height, I'd imagine you could make it fit and have it be safe if maybe uncomfortable. Worst case, you could always just not wear a space suit for the trip home. You only need the suit in the case where the capsule depressurizes but still survives reentry. To my knowledge this has only ever happened once. If they really need to leave the space station and there are available seats, this seems like a no brainer. It would be like evacuating from a tsunami in a car that didn't have seatbelts.