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

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