Well, yes and no: While you can have two different portal-ext.properties, they all will be adding to each other. A single installation of Liferay will access one database, with the notable exception that you can configure "database sharding" for multiple Liferay instances - but you don't need multiple portal-ext.properties files for that.
If you're just asking how to use a second database for your business specific functionality (e.g. your own application's database aside of Liferay's database), you can connect to that through whatever means you want - Liferay doesn't have to know of it at all.
