What does Donald Trump's rule, “There can be only one No. 1, Two-Face.” mean?

Maureen Dowd’s article in today’s (May 7th) New York Times that came under the headline, “Donald Trump or Paul Ryan: Who’s King of the Hill?” portrays the argument between Paul Ryan and Donald Trump, and wraps up with the following paragraph:

“For now,” Trump murmurs, taking out his hair spray for a spritz before he walks past the press octopus. “What I will remember is that you sabotaged me when I should have been savoring my success. And you should remember the No. 1 rule from “The Art of the Deal”: There can be only one No. 1, Two-Face.” (From The New York Times.)

I can’t understand what the last phrase, “There can be only one No. 1, Two-Face” means. Of course, there is only one No. 1. There cann't be two No. 1s, but what is “Two-Face,” and it’s not “Two faces”?

Perhaps better punctuation will help:

Trump murmurs... "What I will remember is that you sabotaged me when I should have been savoring my success. And you should remember the No. 1 rule from The Art of the Deal: 'There can be only one No. 1,' Two-Face."

In Dowd's fevered mind, Trump is talking to Ryan, whom he addresses as "Two-Face" after telling him about his rule There can be only one No. 1.

Trump means that Ryan may be the highest-ranking Republican elected official now, but that he [Trump] means to replace him [Ryan] in that role by becoming President.

"Two-Face" is a vocative and in the tradition of Trump, it's a derisive nickname. Two-Face means deceitful.

Two-faced is an insult meaning insincere and deceitful. Calling someone "Two-Face" most likely refers to this.

Also, see Two-Face, a villain from the Batman comics.