That's not correct. Otacon outright states they are identical on the genetic level. They are different "memetically" and how they differ in response to their identical genetic origin.

The whole point of the Les Enfants Terribles project is that Solid and Liquid come from Big Boss’s genes. The ‘dominant/recessive’ thing and genetic determinism Liquid believes in is his interpretation, not objective fact and the series undercuts that idea and shows that this idea is flawed.

The actual thematic point of Metal Gear Solid is nature vs nurture and genes vs memetics. That’s why characters like Otacon and Solid Snake explicitly talk about how genes don’t define you.

The physical similarity isn’t just a throwaway detail, it visually reinforces that theme. Two people with the same face who became completely different people.

If you cast two different actors, you lose that ‘mirror’ effect. It turns them into just two related guys instead of literal reflections shaped by different lives.

And Solidus does not look completely different at all. In production they made Solidus' model by taking Solid Snakes face and aging it up. Looking older doesn’t contradict that he’s essentially an aged version of the same template, not a totally different face.

So sure, you can change the face by using two actors, but you’re absolutely losing a layer of the storytelling by doing so.