Есть два пути решения вашей проблем

Есть два пути решения вашей проблемы.
1. Сменить тип данного поля на checkbox и тогда пользователи в search форме могут выбрать несколько полей одновременно, а на добавлении листинга сделать лимит на выбор только одно поля с помощью изменения кода.
2. Оставить тип поля, а в search форме изменить код и сделать так чтобы пользователи могли выбирать несколько раз из списка.
Оба варианта будут платными.

Результаты (английский) 1:

There are two ways to solve your problem.1. To change the type of this field to check box, and then in the search form users can select multiple fields at once, and to add a listing to limit the selection of only one field using the code changes. 2. Leave the field type in the search form, and change the code and to enable users to select from a list of several times. Both options will apply.

Результаты (английский) 2:

There are two ways to solve your problem.
1. Change the type of the field on the checkbox and then users in the search form can select multiple fields simultaneously, and adding a listing to make a limit on the range of only one of the field by changing the code.
2. Leave the type of field, and in the form of search and modify the code to make sure that users can choose from a list of a few times.
Both options will be paid.

Результаты (английский) 3:

There are two ways to solve your problem.
1. Change the type this field on the checkbox and then users in search form may select multiple fields simultaneously,And the appendix listing to do limit on the selection of just one field using the code changes.
2. Leave a type of field, as well as search the form you want to change the code and to do so to allow users to select from the list a few times.
Both options would be pay-per-view.