You don't really need to break up with Miranda. Once you commit to a love interest all other romanced characters will revert to the friend dialogues.

I've done this with Garrus and Kaiden:

First: I talked to Kaiden before committing to a romance and picked the break up option and dialogue really went sour between them.

Then I re-loaded and waited until I got Garrus' commit scene and agreed to a relationship with him, then I went back to Kaiden and we had the friendly discussion being content with being friends.


So I think the game just switches flags of characters you romanced once you decide on one and all others revert to the just friends style dialog.