If we have two discount groups and we tag the customer in both, the app applies flat/advance shipping rules of the discount group with a higher discount. For example, I create two discount groups that apply to the same customer tag 'Wholesale'.
One of them has a Discount percentage of 50% and a Flat shipping rate of 26$.
The other discount group has a Discount percentage of 60% and a Flat shipping rate of 22$.
As you can see, a flat rate of $22 corresponding to the higher discount group (60%) applies to a customer tagged with Wholesale.