So, I was in some junky store called Fred's. I was looking at their trading cards (still need some Mario Kart Wii cards) when I found this gem.
Several odd things about this.
First, the other 2 for 1 packs like this were things like Yu-Gi-Oh, Pokémon, some animated movie themed cards, etc. This was the only one of these I found.
Second, these cards (as far as I know) were never sold as just packages. The Foil Packs were always on cardboard-blister packs.
Third, this 2 for 1 pack actually contained three packs. Oddly, there were two packs of Baseball, one pack of Tennis. I'm sure I saved some poor little kid some tears.
Finally, Baseball was released in NES series 1 packs. Tennis was series 2. I even had EasyCure double check for me (turns out, there's this awesome card list for the e-Reader on GameFAQs.).
Best $1 I ever spent.