Question RE Dual forecasts

[fintron]

A quick question for you footy buffs if I may? How do the bookies work out the odds for a dual forecast? Is it as simple as multiplying the individual qualifcation odds of the two teams together, i.e, in Group D England are France are both 1/2 to qualify. Is it then a case of 1.5 x 1.5 = 2.25, i.e 5/4 for the dual forecast, or is there something else that needs factoring into the price because the selections within a group are related? Reason I ask, on William Hill Euro slips it says minimum odds for backing 8 teams to qualify is 18/1 - that is the cumulative odds backing the eight shortest priced teams to qualify across the four groups. However, if you work out a four fold comprised of dual forecasts for those same eight teams (bearing in mind it doesnt matter what order they finish in for the purposes of a dual forecast) the odds are significantly bigger. Not all bookies allow you to multi up dual forecasts (william hill for one) but others do. Cheers in advance EDIT: a mate just pointed out to me that once one team has qualified, the odds on the second team qualifying are effected since there's one place up for grabs and three teams competing for it.