The solution that I have found includes using the 'row' and 'span' classes provided by bootstrap and inserting them into the various fields using the 'wrapper_html' provided by simple form.
Example:
.container
...
= simple_form_for(@stuff, :html => { :class => 'form-horizontal' }) do |f|
= f.error_notification
.form-inputs.row
= f.input :name, :wrapper_html => { :class => 'span6' }
= f.input :contact_name, :wrapper_html => { :class => 'span6' }
.form-inputs.row
= f.input :contact_email, :wrapper_html => { :class => 'span6' }
= f.input :contact_phone, :as => :string, :wrapper_html => { :class => 'span6' }
You can read up in the Docs about bootstraps grid system:
http://twitter.github.com/bootstrap/scaffolding.html
Or check out some examples of bootstrap and simple form integration:
http://simple-form-bootstrap.plataformatec.com.br/articles/new
Hope that helps!