Use the Computed Field module to create yet another field stored in the database, combining the two (or more) fields you want to search, then expose a filter for that field instead of both of the individual fields. (I don't like this because it duplicates content/storage, and involves an extra module to do so).

Use the Views Filters Populate to invisibly populate a second field that you've added to a views OR group (using Views OR in Views 2.x, or the built-in AND/OR functionality in Views 3.x). (This module is slightly limited in that you can only work with strings, and again, it involves an extra module).

Implement hook_views_query_alter() in a custom module. In the query alter, you'll simply get the keyword parameter, and add a join and where clause (if you want to join to another table, like the 'body' data table).