3.x do not support this feature. What we could suggest.
1. Use domain mapping - very useful solution that event does not require second installation for the subdomain. More information at Domain-Page mapping
2. Create a new index in the subdomain - say "Pages". One the website is indexed get the folder that is generated under Sitefinity/App_Data folder called Pages. Afterwords copy this folder to the same location of domain project.
Note that you should have a search index called "Pages" ( with the same name) in the domain website. So you just get the segment files from the subdomain and paste them to the domain search folder.
Then create a custom SearchResults control, override CreateChildControls method create two generic lists of type ResultItem, merge the both list in a new one say NewDataSource and bind the SearchResults repeater to this
3. The Telerik.Lucene.Net to make your custom logic - provider and indexer to merge the subdomain/domain segments.
the Telerik team