What is a same-game parlay worth
Why can't you just multiply the legs together?
Multiplying the legs is the right sum only when the legs are independent. Same-game legs are not.
A team covering the spread makes the over more likely, not less. A quarterback having a big passing day makes his receiver's yardage prop more likely. Multiply those together as though they were coin flips and you get a probability that is too low, which makes the fair price look higher than it is — and makes the parlay look better than it is.
These legs are in one game and are not independent — a team covering the spread makes the over more likely, not less. Books price same-game parlays with a correlation adjustment for exactly that reason, so the independent figure above is an upper bound on what this is worth, not an estimate of it.
Books price same-game parlays with a correlation adjustment for exactly this reason. That adjustment is theirs and it is not published, which means the independent calculation you can do is a ceiling on the value, never an estimate of it. Any tool that quotes you a same-game parlay edge from multiplication alone is quoting a number it cannot support.
What survives: correlation cuts both ways, and a negatively correlated pair is the one books misprice more often. That is a real edge and it needs the correlation measured, not assumed.