Redmine/app/views/issues/_form_custom_fields.html.erb

<% custom_field_values = @issue.editable_custom_field_values %>
<% custom_field_values_full_width = custom_field_values.select { |value| value.custom_field.full_width_layout? } %>
<% custom_field_values -= custom_field_values_full_width %>

<% if custom_field_values.present? %>
<div class="splitcontent">
<div class="splitcontentleft">
<% i = 0 %>
<% split_on = (custom_field_values.size / 2.0).ceil - 1 %>
<% custom_field_values.each do |value| %>
  <p><%= custom_field_tag_with_label :issue, value, :required => @issue.required_attribute?(value.custom_field_id) %></p>
<% if i == split_on -%>
</div><div class="splitcontentright">
<% end -%>
<% i += 1 -%>
<% end -%>
</div>
</div>
<% end %>

<% custom_field_values_full_width.each do |value| %>
  <p><%= custom_field_tag_with_label :issue, value, :required => @issue.required_attribute?(value.custom_field_id) %></p>
  <%= wikitoolbar_for "issue_custom_field_values_#{value.custom_field_id}", preview_issue_path(:project_id => @issue.project, :issue_id => @issue.id) if value.custom_field.full_text_formatting? %>
<% end %>