Yii Using mongo DB and MySQL at the same time

I look at how to create a system for working with native languages, tags and such data in the Yii Framework.

I have already chosen MongoDB for storing my data, since I think it will work fine and get better performance at lower cost (there will be a huge amount of data in the database).

My question is about user authentication, payments, etc. These are sensitive bits of information and areas where, I think, relational data.

So: 1. Will you use two different db systems? Should I need them, or am I complicating this? 2. If you recommend a two db approach, how can I achieve this in Yii?

Thank you for your time!

PS: I do not assume that this issue will be another endless discussion between relational and non-relational people. Having said that, I think my data is about mango, but if you have something to say about it, then :)