Two thoughts:
1) there is a big difference between buying the DOW alone, and {selling} the S&P-DOW spread.
2) working the spread (not so ubiquitous a US thing as overseas), step one is to make sure you've got close-to-identical notional values -- e.g., trading $5k of S&P against $5k of DOW.
Okay -- a free additional 3) Many brokers will allow you to create a tradable contract, like "S&P-DOW" -- that will make graphing, management, but possibly *not* {tax} accounting, lots easier.