inertia_trader, while I appreciate your reply, I don't think it's correct: let's consider two 100% correlated instruments with a spread of say 5 points between them. That is, all the time the spread is constant: 5 pt, apparently, volatility of the spread is zero, so the call on this spread should also cost close to 0. Which is not the case when you buy call and put as you suggested (as volatility of each pair constituents can be high).
So there must be both buying and shorting options... If it's possible theoretically at all.