Correlation
Correlation
...defined as the mutual relation of two or more things, parts, etc. is discussed by Stanford Wong in his book "Sharp Sports Betting" and in the excellent article by Nolan Dalla.
Correlation is rare: the term "correlated" only really applies in certain limited situations (e.g., where two independent propositions win because they share one determinative event in common, like when Superbowl props offer "longest field goal" and "only one field goal" bets, the only fg will be the longest).
Correlation also occurs when the same wind storm will affect two different football games in the same region of the country, thereby keeping the score of both games under the number.
IMHO, the only things worth parlaying are these situations. The situations you are suggesting here, gjn23, are not really correlations (more like quasi-correlations). This is not to say that they don't have tendencies to happen together, like the following:
dog and the under, or favorite and the over