You are looking for this:
Warhammer 40,000 RULEBOOK Official Update Version 1.6 wrote:Q: Some abilities can cause extra hits or wounds on a speci c roll e.g. ‘Each hit roll of 6 scores 2 hits instead of 1’ and ‘Each hit roll of 6 scores 1 additional hit on the target’. Are these abilities cumulative?
A: It depends on the exact wording of the rule in question. Rules that ‘score 2 (or more) hits instead of 1’ are not cumulative. Rules that ‘score 1 (or more) additional hits’ are cumulative. Here are some examples to show how these rules interact:
1. If a model is affected by two different rules that say that on a hit roll of 6 that attack ‘scores 2 hits instead of 1’ then a hit roll of 6 will still only result in 2 hits being scored against the target unit.
2. If a model is affected by two different rules that say that on a hit roll of 6 that attack ‘scores 1 additional hit‘ on the target, then a hit roll of 6 will result in 3 hits being scored against the target unit.
3. If a model is affected by two different rules, one that says that on a hit roll of 6 that attack ‘scores 2 hits instead of 1’ and one that says that on a hit roll of 6 that attack ‘scores 1 additional hit‘ against the target, then a hit roll of 6 will result in 3 hits being scored against the target unit.