Merging ChangeSets

1. Developer A adds a first ChangeSet with <addColumn tableName="ARTICLE"> to add a column
2. Later Developer B adds a second ChangeSet with <addColumn tableName="ARTICLE"> to add another column to the same table
3. Before pushing these changes to production, the DBA calls for merging these 2 changes in a single one because this ARTICLE table contains a large number of rows. As a result, this changes needs to be optimized.

How can we do that with Liquibase?

Using preconditions, I can do something to prevent ChangeSet 3 from running if ChangeSet 1 or 2 have already been applied. On the contrary,  how could I prevent 1 and 2 from running (and let 3 run) when neither 1 nor 2 have been applied?

Gérald