What I do is re-rate the odds from a (average) of 118% back to a 100% market. Then you convert the odds of your first horse into it's chance of winning(this assumes the market is usually right) then for your second horse you do the same but take the first horses chance of winning the race from the 100%. Then you multiply the chances of the two to get the probability of them running 1st and 2nd in that order. Do the whole thing again but for the other quinella combination and add the two probabilities and convert back to a price. You can see why I wrote it in excel!
I'm happy to email the equation, ask me at ewan@phaedrus.com.au.