I don't believe that this is a primary KPI to earn in designing a form.
During several usability tests (qualitative and quantitative) we noticed that users are much fast and good to accomplish the task when the form is built according to the human behaviour.
For instance, when you present yourself you say "hello I am John Doe" and not "hello I am John Adam Doe".
If for some reasons you need to record the optional second name, maybe, you should think about a full customer page where you can display all the fields that you need. In an entry form, I don't see any added value to go so deep into the details.
It's because I am working on an Android app and the style-guide for such OS presents the form controls with this design, I've to say I'm not a big fan of that (I'm not a big fan of Android at all)