Example
Long NQ at 1% risk and long ES at 1% is effectively 2% on one idea.
In practice
Before opening a second trade, traders ask whether it is really a different idea. Stock indices move together, most crypto follows Bitcoin, and many currency pairs are driven by the dollar.
The practical rule is to treat closely correlated positions as one trade and share a single risk budget between them, or to pick the cleanest chart of the group and trade only that.
What to watch for
Correlations change over time and tend to rise sharply in a panic, exactly when diversification is needed most. Assets that normally move apart can all fall together.