You can use dbms=“oracle” and dbms=“mssql” in your changesets, so you have 2 times the same changeset, one for oracle and the other for mssql. This case is when you have or that contains specific code for a given DB. If you use you can have just 1 changeset with the table creation and will work for both databases.
Basically, if you can use liquibase refactorings you can share changesets, if not you put for which specific dbms you want to run the changesets and only on that db they will be exceuted. But you always run liquibase using the same input xml file.