Search in section only
You would just pass the other value (i.e. the section handle) in to that template as GET or POST parameter and in the search results template grab the paramater and use it, as in your code.

If using GET, the url you'd construct would be something like:

http://whatever.com/search?query=blah&amp;sectionHandle=blah2


or with POST you might use e.g. a radio for the section option with the name being sectionHandle and the value being the actual handle of the chosen section.

Either way, in your template you then just:

{% set sectionHandle = craft.request.getParam(sectionHandle) %}
{% paginate craft.entries.search(query).section(sectionHandle).limit(5) as entries %}