Spitty is accepted in Chihuahua, though it's not in all dictionaries. Wiktionary includes spitty, and specifically states that the comparative and superlative forms are more spitty and most spitty. It's hard to tell whether people mainly write spittier or more spitty, because both are so seldom used.
In Letters and Numbers, the Australian version of the TV game show Countdown, The dictionary guy, David Astle, used to disallow any comparative or superlative that required a spelling shift (such as y to i), unless it was specifically listed in the dictionary they used. Since I can't see spittier listed in any dictionary, I'm not persuaded to add it.
