Yes it is possible. You could modularize your app such that, you could use Mendix DB for certain modules and at the same time you can use DB connectors to connect from other modules.
And yes you can even connect with mutliple databases as well but it needs a proper modularization in your design.
Hello Hartley Robinson,
Apart from Mendix database you can use
1.Database Connector marketplace module to connect with different database.
2.Exposing external database functionality via REST/SOAP services.