Well I've been thinking of this for months and I finally decided to make a blog about it.
Here's my theory about BB's ability to have 2 Devil Fruits:I remember that we never saw BB eating the Yami Yami no Mi.Instead of that,we saw him killing Tatch.But we didn't see BB eating or taking it from Thatch's dead body,so I think That was not the first Devil Fruit Blackbeard ate.I believe he had eaten A devil fruit before that which gives him the ability to take another DF's DF after killing him.So when everyone went to sleep he convinced Thatch to eat his DF and as soon as he did,BB killed him and took his powers.He did the same with WB too.After killing him,he took his Devil fruit.I don't know the process he has to do to acquire the df he wants but I'm definitely sure that's how he does it.
Οk,that was my blog and I want to hear your opinions.