Hi PeterRegg,
In theory, the only way to keep career progress separated is to disable Steam Cloud for Planet Coaster. This will disable saving saved parks, blueprints and career progress on Steam Cloud and thus disabling the synchronisation of this between computers and user accounts using the same Steam Account.
Once disabled, you should be able to use 2 separate Windows user accounts on a computer, or 2 unique computers, to keep the saved parks, blueprints and career progress separated.