This can be done by adding length or width.

OR by increasing displacement.
Singles stay upright because the paddler is always subtly correcting for "tipping" It is the same way, essentially, that a human can stand. You are always correcting to avoid toppling over.

With two people, this is not so straight forward. If all one needed, in designing a double, was to increase displacement you could just design it deeper.

I think it primarily for reasons of stability that a typical double is much wider. There are plenty of us who are experienced paddlers and paddle both but still prefer a double that is wider than our single...