Its not possible using the out of the box SPD workflow  actions. However,
you could develop a custom action for SPD using Visual Studio that would
allow you to access information in a list  in another site. Check CodePlex
for similar actions already developed.

We've "sort of" done something very similar, not with 2 sub-sites, but with
objects within one site  that we "think of" as two sites because the
permissions on the two sets of objects are mutually exclusive. We then have
WFs that combine the data  from the two sub-sets into one common set which has
general access to many users.