Displaying two rows is quite difficult to achieve as it has to do with how big the font is: what would be one row on one pc could be four rows on another. Limiting the length of the announcement would be a better requirement. You could always try to play with the settings a bit to get a string which would in general be about two rows long, that's as far as you can get.
From your question I assume you mean a content aggregation. If you're using the Content Query Web Part you could easily do that in XSLT using the substring function, like: