You just have to make sure that each detachment contains only models from one faction. So you could take a Deathguard detachment- say a brigade. You could then take another detachment of Daemons- maybe a battalion.
If, however, you put a single daemon unit in your deathguard detachment or a single deathguard unit in your daemon detachment, you lose battleforged.
Don't listen to this it's wrong/misleading information.
Eldar, chaos, imperium, Tyranids are the keywords you can't use for detachments. Every other faction keyword is fair game.
Feel free to use nurgle and tzeentch but take note the penalty is simply not getting your detachment bonus and no access to stratagems (unless you have another detachment that unlocks them)
Only some deathguard and thousand sons units care about there chapter tactic anyway so seeing tzeentch/nurgle battalions etc is common in competitive play.
So in a 2k list you could have a nurgle vanguard , a deathguard battalion and a nurgle deamons supreme command and use both deathguard and deamons stratagems
If you had a deathguard and a nurgle detachment you could only use deathguard stratagems for example even though you have plaguebearer and what not in your list.