Let only one player needs a copy of the DLC to play it.

Similar to what’s done in Minecraft bedrock, in the minecoin shop. Where if one person buys a modpack or world, their friends don’t also need to buy it to join. They just join and it’s installed at the loading screen. Plus it’s not like the DLCs are that much of an install either, but if that’s a concern, just do what It Takes Two did, and have it preinstalled, but need a host in order to play. In short, make online co-op more like couch co-op without the split screen of course.