I'm still waiting to see how that plays out. We still haven't really had an explanation as to what those with the name of "D" have that is different than other people. I know Blackbeard can have two devil fruits, maybe luffy can as well. But it still wouldn't explain how he can add fire element to his attacks unless he got ace's power. I remember back in the chapter where Caesar's poisonous slime monster died and nearby fruit turned into a devil fruit, but maybe when a devil fruit user dies the nearest person with "D" in their name inherits that power. shyts confusing as fukk I guess we jus gotta play the waiting game till Oda explains it all.
I'm not sure WHY Blackbeard was able to get two DF powers without dying, although Marco mentioned that his body was weird, so maybe he has two stomachs or something like that, But the HOW is easy to figure out if you read between the lines. If it was a matter of just getting the power because he was a "D" then he wouldn't have needed to cover up WB's body for a minute.
Think about what we know -
1. As far as we can tell, you still have to EAT a Devil Fruit to get a power regardless
2. We've seen a nearby fruit turn into a Devil Fruit when a user dies
3. Oda has said that the same Devil Fruit powers doesn't exist more than once at a time
So, if you don't get it yet, what I'm saying is that
When a Devil Fruit user dies, the nearest fruit turns into the Devil Fruit they ate
So what does that have to do with Blackbeard? It's actually really simple.
Blackbeard had a piece of fruit with him, and intentionally wanted to be around when Whitebeard died so that his power would transfer to the fruit. That's why he said "I'm glad I made it in time to see your death!!". He just ate the fruit he had with him right after WB died. The reason the crew covered Whitebeard's body and Blackbeard with a cloth was so that the crowd wouldn't see him eating the fruit and discover how he stole his powers. It's the Blackbeard Pirates' little secret, and it's how the BB pirates are stealing DF powers in the New World.