It's about 1 3/4 miles between the two - how about a rather swift walk! Maybe just leave the Benfica game 5 mins from the end if you see that the score wont change, and you will be one the first to leave the stadium, jump on a taxi to Alvalade stadium which is not that far, just over a mile.
The Derby de Lisboa or Drbi de Lisboa (English: Lisbon derby) is the biggest football derby match in Portugal. It is played between Lisbon-based clubs Benfica and Sporting, two of the most decorated clubs in the country.