If you want to have registration page divided into several pages, just add a new account type in yoursitename.com/admin/questions/edit-account-type. Once the new account type is added registration page will get divided into two stages: base questions and exclusive questions.
make the second account type invisible.
remove “Select Account Type” field completely from .html files since you don’t seem to need it?
The file to remove this field from “Join” page is join_index.html (ow_system_plugin/base/views/controllers).