If a product has overlapping discount rules, it will use the earlier rule first and only switch to the next rule after the first rule expires. You can refer to the diagram below for a clearer illustration;
If Product A has two Sales Campaign overlapping at 30/06/2020, the 25% discount will be applied instead of 50% at that date as Sales Campaign A is the earlier rule. The discount will change to 50% on 01/07/2020 once Sales Campaign A ends.
IMPORTANT NOTE: If you have overlapping periods for the same product and you disable the earlier discount rules, the discount for the product will be disabled.