I think the main problem is handling the money and if the devs are not friends, one of them will have a feeling that the other one is riding on their popularity.
There are some cases where both would be equally popular or a similar genre. If one is more popular, it doesn't have to be a 50-50 split. It's even more powerful for new developers with no portfolio, who are 'higher risk' to the buyer.
Heck, it could even be a 100-0 split or something if one of them actually wants to ride on another's popularity. One gets marketing to the right market, the other gets to charge more for a game. Yeah, the one who's not getting a cut won't make money, but especially for new developers, you'd want to build a fan base.
Also, there are tons of people who want to pay 30$ for game A, but 0$ for game B.
Not an issue because they can still buy it separately. It's there to give an option for those who do want both.