Joint Assault is an Instant from Avacyn Restored. For one Green, it gives a target creature +2/+2. If that creature is paired with another creature it gets an additional +2/+2. By paired, it means two creatures paired using the Soulbond mechanic. There were some seriously played Soulbond creatures at one point in Standard: Lightning Mauler, Silverblade Paladin, and Wolfir Silverheart. (Wingcrafter was played a bit when it was first released.) But outside of Lightning Mauler seeing some fringe Modern play in aggro decks, Soulbond isn’t really a part of the competitive scene anymore.
Yet we have this little pump spell that has been pretty much completely forgotten about. Avacyn Restored wasn’t known as one of the most fun Limited sets. However, Soulbond was definitely a cool mechanic, and all of the Soulbond creatures had some playability in Limited. But for players to still be playing a deck primarily full of Soulbond creatures, it would have to be some sort of 2 or 3-color Human Tribal deck, with a copy of Wolfir Silverheart thrown in for good measure. Or it could be straight Red/Green aggro.
I’m not quite sure what this list would look like, as there’s a bunch of ways you could go, especially in kitchen table Magic. It’s just really unusual to see a common with such narrow playability be among top selling cards anywhere.