With Instagram, you can search either for a user or a hashtag, but you cannot search for photos from a single user that feature a particular hashtag. You can use a separate search engine to try to find the images you want or you can view the hashtag's RSS Feed and search within that page for a given user.
Using a Relevant Search Engine
Although not endorsed or certified by Instagram, some search engines are made to use the Instagram API to help users find what they are looking for. Visit OnInStagram, Webstagram, Statigram or another free search engine of your choice. Type the text of the hashtag in the search field, leaving off the hash or pound sign. In the Find dialog, type the username, which will highlight all the corresponding entries in the search.
Viewing a Hashtag's RSS Feed
Instagram provides an RSS Feed associated with every hashtag. In your browser bar, paste the link to Instagram's generic RSS Feed (see Resources) and replace "[hashtag name]" with the actual text of the hashtag you are searching for, leaving off the hash sign itself. Open the Find dialog, enter the username and all relevant entries will be highlighted. The URL that precedes each username entry is a corresponding link to view that particular image.