Making taxonomy dropdown on the basis of previously selected dropdown of two different taxonomies? I have two different taxonomies
Taxonomy 1:

Category 
-music 
-football


Taxonomy 2:

Sub-category
-Jazz
-Rock
-Fifa
-Russian Open


I have linked Jazz, Rock to music and
Fifa, Russian Open to football by term-reference field.

I have content type event, having term-reference to both Category and Sub-category. I want drop-down for sub-category should be populated after selection of Category. 
Eg. if we select music as category then in sub-category only jazz and rock should be shown.
I think you will have to go for some custom code if you want to use seperate taxonomy vocabularies.

An easier way would be to have all terms in the same hierarchical vocabulary and use Hierarchical Select to create the "populate dropdown"-feature.