I believe that the Datafeed Toolbox only allows specifying one symbol at a time. See here for example.
As an alternative, consider using my IQML (IQFeed-Matlab) connector. IQML enables both synchronous (blocking) and asynchronous (background) queries for multiple symbols, that are fetched either serially or in parallel (using the Matlab Parallel Computing Toolbox). A simple usage example:
data = IQML('history', 'symbol','XIV,AAPL,MSFT,GOOG', 'UseParallel',true);
IQML was developed with top performance, reliability and usability in mind. IQML supports 100% (repeat: 100%) of IQFeed's API functionality. It is fully documented, continuously maintained/improved, and I am happy to provide support.
Note: I am an independent software developer and not a MathWorks employee. Don't get angry at me for bringing an independent alternative to the table...