I have a Wordpress site. I want to search with 2 input text field in one search form. One for searching title and one for custom-field.
First, you'll need to create a searchform.php file in your theme with the new field. It's best to create a child theme for any custom work.
Once you've modified the search form, you'll need to hook into the search query. I don't know off hand what filter you'll need to use, probably the pre_get_posts hook. There's some good documentation about how to use pre_get_posts on wp's codex.