Multiclassing from 2e, at least the one campaign I was involved in, was similar to the gesault in terms of levelling in both classes at once, so that does seem what they might be referring to in terms of dual classing. Should be interesting how they translate something like that into 4e ...
Of course, considering that backgrounds are effectively optional rules introduced in PH2, it's possible that this is another optional rule, and thus very similar to gesalt in terms of getting everything of each class, but don't stack them (i.e. take better of hit points, number of skills received, etc) and then choose from the combined skill list. If it was meant as a 'core' rule however, it would need to be balanced against normal classes.