Groupsave to be fair, targets those families who would struggle to get value against the car. I see less justification for the duo offer, but then it's availability is so limited, it doesn't really register with me at all.
Except GroupSave is usually not worth it for families because it only discounts adult fares and only on groups of 3+ adults, whereas a Friends and Family Railcard discounts adult fares by the same amount and also discounts child fares by 50%. For a 2 adult 2 child typical family, GroupSave isn't even an option unless you intentionally buy a child and adult fare. For the F&F Railcard to make its money back the average fare saving needs to be a mere £8.75pp on average in this scenario, which is usually done on one trip. Guess how many parents take a look at rail fares without the F&F Railcard discount and think nah stuff that I'll drive though.
I doubt many groups of adults even know about GroupSave. In fact I've been on forum trips where we might've been able to save money with GroupSave but we haven't. It's a bit hard when so many forum members get staff travel etc though! I've certainly taken people on hiking trips and only remembered about GroupSave afterwards instead of buying a couple of Two Together Railcards. And I'm somebody who knows more than the average person about ticketing. Without me in that group they won't have even got the Two Together Railcard discount, and may even have just driven.
The added complication with GroupSave and the F&F Railcards is a lot of the operators also offer Kids for a Quid tickets, like EMR, Southeastern, and ScotRail. At which point you're probably best off getting a Two Together Railcard for the two adults then Kids for a Quid tickets for the children.
Southeastern even point this out on their GeoupSave page, except they fail to mention the Two Together Railcard:
If there are less than three adults in your group, however, it may work out cheaper to buy regular Off-Peak Tickets for the adults and Kids for a Quid Tickets for the children.
Doesn't it all just sound absurd when you start processing it.