As far as I know (and I’m no lawyer or expert) the Spanish do not allow you to have dual citizenship. My British friend got Spanish citizenship and had to hand in her British passport at the time and swear allegiance to the Spanish flag. Of course, the next day she reapplied for her British passport and as the British do allow you to hold more than one passport, at the consulate they just marked down ‘given up to Spanish authorities’ or something similar on the application form and happily gave her and her kids another one. Apparently it happens a lot.
My advice - swear to be a good Spanish citizen and then do whatever afterwards!