This question --allegedly sarcastic-- was asked by Spain’s Foreign Minister García-Margallo. (It’s significant that the Spanish Minister dealing with the Catalan case is none other than the Foreign Minister!). Margallo asked the question to persuade liberal and centrist voters not to vote for the independence list headed up by Romeva, the former MEP from ICV (1).
But in another way it was also asked by Joan Coscubiela (2) when he claimed that Romeva suffers from amnesia for appearing in the same electoral list as Mas, in this case so that voters on the left wouldn’t vote for a list that proposed Mas as president.
The answer to the question is simple: because both sides think that what unites us is now more important, even if it’s just for the one time, than what divides us. That the common goal of achieving independence is more urgent than opposing ideologies. Something similar to what the people thought when the all-embracing Catalan coalitions Solidaritat Catalana and Entesa dels Catalans were created in the last century.
And those who think that this shared goal --to build a new State-- is wrong or irrelevant, cannot understand. (That’s why it would be a mistake to start a discussion on who might be president, if the joint list were to win. To bring up the question drives voters away. And it allows Margallo to think: don’t you see how they can’t run together? They’re at each other’s throats at the first opportunity.)
(1) N.T. Iniciativa per Catalunya Verds (ICV) calls itself an "eco-socialist" party but in some ways it continues the legacy of Catalonia’s old communist party. Romeva quit ICV a few months ago after his party chose not to support Catalan independence.
(2) N.T. Joan Coscubiela is an ICV MP.