I think its too late. Macri ran on "we will not work with anyone who was part of the current admin and Massa is worse than the devil himself". What choice does he really have now? Turn around and say sorry? I think Macri and Massa should have been together. I don't think legally they can do anything anymore (the tickets are set as they are). What deals are they going to cut?
But other than all that legal and backroom stuff, I really don't see people who support one forgetting their differences to support another candidate. Sure they are "anti kirchnerista" but as nice as that sounds, what position do you really have when you say, "I am not them"? Which is what Macri was hinting at that everyone who is anti kirchnerista should vote for him. Well support based on nothing for and just against the current government is bound to result in every individual selecting his version of Peronismo to back in the elections. You have kirchneristas because they support Kirchner policies (its a different matter whether those policies are good or bad). And then you have the opposition who is voting against Kirchner. If everyone got behind Cambiemos from the get-go (like PRO, UNEN, Radicales) then I could see them having a huge advantage right now.
But as things stand, after the first round of voting, if there is balotaje, I think everyone should get together to throw their support behind whoever is in the second place. Because unless the poll numbers are extremely inaccurate, I don't see anyone from the opposition getting more votes than Scioli in the first round.
Throw their support behind the second guy and make sure they only focus on things that are common. And the positive ideas (not just anti Kirchner rhetoric) should be what they should focus on. From the debate it didn't look like all of them supporting one candidate should be that hard to do anyway.
Like Gringoboy said, too many opposition parties, they should have only had one. What a wasted opportunity!
Other than this I am not sure anything can really be done but I would be happy for someone to enlighten me further on this issue.