As to what the 140 now 280 means, I think he will keep the @realdonaldtrump, plus the @POTUS one.
“...“I can put out Twitter — and it’s not 140, it’s now 280 — I can go bing bing bing.” This could mean a variety of things:
A. Donald Trump has a secret Twitter account that allows him to do 280 character tweets.
B. Trump will also use the @POTUS account, and thus a @realDonaldTrump tweet + @POTUS tweet = 280 characters....”
I’ve noticed that a lot of his tweets spill over into two tweets. I think that’s all it is.