Co-op multiplayer in Wasteland 3 will not feature Xbox One and PC crossplay but that is the only bit of bad news as the rest sounds rather flexible, which is a good thing.
Microsoft and inXile Entertainment revealed the particulars of Wasteland 3's co-op. While both players will have to be on the same platform, the rest of their adventures together will be flexible as they offer many options on the go.
For example, you don't have to create a new game that will be explicitly multiplayer. When the players link up, they can choose to start a new game or load a save. In the case of starting a new game, each of them will get to create their own unique Ranger.
Meanwhile, loading a save that is far enough into the game that the players have recruited new characters into their ranks will cause the host to go into squad management screen where they can choose who controls which characters. The number of controlled characters can be asymmetric or both players can control an equal number of them.
One interesting difference between singleplayer and multiplayer is dialogue. When there are two players around, they can independently talk with NPCs and possibly trigger more than one branch of a quest which could result in additional rewards.
If both players are near an NPC that is engaged in dialogue with Rangers, they can transfer dialogue authority to each other.
Finally, one of the most interesting bits is that two players can spread rumours about each other at the home base which should make for some interesting scenarios.