Let say you have two documents, for people and auction then to get the buyer name you have lots of options. Here are a few of them:
Assume you need only the name for a rather small subset of people. Create a view that outputs the PersonId as key and Name as value, then query the view for a specific name each time you need it.
Assume you need many people joined to many auctions. Download the full contents of the basic index from #1 and execute the join using linq.
Assume you need many properties of the person, not just the name. Download the Person document for each auction item.
Assume you need a small subset from both Auction and People. Index the fields from each that you need, include a type field, and emit all of them under the key of the Person. You will be able to query the view for all items belonging to the person.