In 30k, yeah, they teamed up - on occasion to suit their own needs and rarely more than Marine Squad needed help with Event A and Chaos Squad did too. There were never any formal alliance between the two - and should their higher ups (mostly Imperium) have caught wind of it, you can be sure the offenders would have been punished accordingly. After which they always went back to murdering each other given half the chance to do so.
In 40k there is simply nothing that threatens either side enough to even remotely consider an alliance and their hatred towards each other runs so very deep. There’s nothing to obstruct the Warp and there’s nothing to obstruct the Emperium. Yes, there’s the occasional Tyranids, but they don’t really bother the Warp in which Chaos reside, they are a nuisance to the physical realm and rarely bring more than a single fleet to the table, nothing that the Emperium can’t handle should they care enough.
In short, nope, not happening.
Secondly, it’d be weird to introduce Tyranids as an enemy. They consume Worlds, there’s nothing left after they’re done. Whats our business there? To stop them consuming a world? The campaign length would have to be much longer than all of Vermintide 2 base game + DLC + FLC in order to reflect such an enemy proper.
Tau, much more likely to be introduced. Damn Xeno’s sterilize and/or euthanize anything and anyone that don’t comply which could be done in a 4 map or so campaign (god not another WoM single map fiasco).