some of the designs use two plants sharing some of the...

some of the designs use two plants sharing some of the infrastructure and operate as single large unit while each can also operate in solo mode.

If ALK or the JV partner goes for 20Mt they are most likely to use such option as they can start with 10Mt and add the second module (basically second plant seating in parallel) right next to the first. From memory they call it dual rail plants - two plants operating as one that can be staged/scaled.

These designs work well when a company does maintenance on one as the other can still run so there is no full shutdown of production. Same deal if one breaks down the other one can still operate and acts as failover.
From risk point of view such as single point of failure they share very few such parts but there is still some risk - as with any other design/option.

Pluses and minuses with all options but the biggest risk is who is going to be the JV partner as ALK in my view is not capable of doing this solo - based on facts of how they handle the Tomie expansion which is just a small fraction of whatever size plant will be used at NMPP.