Here are some good reasons why you shouldn't use a first name and last name field.

The structure of a name is not the same across cultures. Users who visit your site will consist of a broad range of people from different countries. Your name field should be culturally inclusive so that no one struggles to fill out your form. With most things in life two is better than one. But when it comes to name fields one is better than two.

http://uxmovement.com/forms/why-your-form-only-needs-one-name-field/