In a recent post to his blog Hakre looks at iterating over iterators (multiples all at once) using either the Append Iterator or the MultipleIterator.
PHP's SPL has two build-in Iterators that deal with multiple iterators at once: AppendIterator and MultipleIterator. In this posting I'll cover both a bit.
He gives some sample code for each - showing how to use the AppendIterator to combine multiple iterators into one set and using the MultipleIterator to attach (not append/merge into one set) multiple iterator objects to a single, iteratable object. He als mentions a "lost" iterator, the DualIterator that never made it out of a SVN repository and into the main codebase.