If you're going to run the database connection using a DBO that's named something other than DBO, you'll need to update your web config to make it aware of this.  

Now to the main point...  

Please be aware that the object qualifier feature has had extremely little use over the past 10+ years.  It's likely to become an unsupported feature that's removed at some point in the future.  

Until then, yes, it works quite well and you can use it.  However, if you're doing this in another database that has existing data in it, you'll want to be incredibly careful to know what database objects are going to change every time you install DNN, upgrade DNN, and the same for every extension before you install/upgrade.  If you don't do this testing before each install/upgrade, you could accidentally permanently delete or change other database objects that were not related to DNN, simply because they had the same name or something. 

Now, it's also important to note that not all third-party extension vendors support the use of object qualifiers.  These would need to be treated slightly differently.  However, the ones I can immediately think of do a good job of putting in their own implementation of an object qualifier.  

In short, yes you can do this, but it's risky and will require a lot more work moving forward.  It would be best to just get another database instance and use that.  If you need anything in DNN to read it, you can add a new connection string to the web config, and then anything that knows how to read and use the connection strings can now perform queries on both databases.