1. It isn't clear that they are equal. They are mostly independent.
They each have separate powers, but it is arguable that they are equal, and instead the executive branch, particularly since the start of WWII, has become the dominant branch of government.
Then why has so much of the Trump Agenda failed? This was my main counter to hardcore Sanders supporters (Who like the MAGA people thought that he could do anything). The President isn't really that powerful. The Speaker of the House has much more power in what gets done in this country.
Which is a good thing. Obama, for example, was unable to get much of anything through Congress after 2010, but still managed to be fairly effective president.