In the past, Sparpreis tickets (1 day advance purchase online, must include at least one leg by an express train - ICE/IC/EC) started at 29€ from anywhere in Germany to anywhere else in Germany (short trips under 250 km started at 19€). A co-traveler (Mitfahrer, up to 4) paid 9€ less. So, if you got a short trip at 19€, a second person would be 10€ or 29€, total. For a longer trip, if you got the 29€ fare, a co-traveler would pay 20€, 49€ for both. A 39€ fare would be 69€ for two; 49€ would be 89€ for two.
Several days ago, I notice something was different. A long trip, which should have been 49€ (29€ + 20€), was 48€. Then I noticed a short trip, which should have been 29€ for two, was 38€.
Apparently, there has been a change to the Mitfahrer pricing structure for Sparpreis tickets. This webpage shows it. It's in German; the pricing structure for the English page has yet to be updated¹. For any Sparpreis ticket up to 39€, the Mitfahrer pays 19€, regardless of the price for the first ticket. So, you would pay more than before for the 2nd person on a 19€ Sparpreis ticket, about the same with a 29€ ticket, and less, 58€ for two if the one person price was 39€.
However, over 39€, there is no price break for the Mitfahrer. The second ticket is the same cost as the first ticket, 9€ more than it used to be.
- Note, under "legal notice" on the English side of the Bahn website it says, "no liability or guarantee can be assumed for the currentness, accuracy and completeness of the information provided." Ultimately, the terms of the AGB (Conditions of Carriage, in German) are what matters.