The net benefit is the value to the couple, regardless of each individual's loss or gain. For taxpayers filing a coupled return, the net benefit is the combined value of both taxpayers' refunds (or balance due).
For example, consider spouses Mark and Carol. Mark owes $150 and Carol expects a $600 refund. Even though Mark owes money, the net benefit to the couple is $450 ($600 - $150).
With expenses that can be combined and claimed on one return (such as donations and medical expenses), the program automatically considers the net benefit, and then allocates the expenses to the more favourable return.