Manipulate only visible fields

Manipulating hidden fields causes "Element is not currently visible and so may not be interacted with" exception. In addition to find() method looking for labels containing text it causes unability to fill a field if another hidden field contains this text.
Here is an example http://data.gov.uk/user/register
I can't fill "Username" and "Password" fields because Mink is trying to fill fields on the first "I have an account" tab.
Hidden "Username or e-mail address" field on the first tab blocks filling "Username" field on the second tab, the same with hidden "Password"  on the first tab blocking filling "Password" on the second tab.