Hey Alex,
Mail chimp only recognizes certain fields:
MailChimp currently supports the following fields. Additional fields will be ignored.
First Name
Last Name
Email
This is from http://forum.webflow.com/t/mailchimp-signup-form-integration/1331
I’d suggest using a single field for the full name but list it as first name only or last name so you can use the other for your specialism field and label it that way so it will come through mail chimp as first name or last name. Other than that, you’d have to use a specialized form or use zapier to somehow automate it. Good Luck!