Dependent Drop Down Forms

I am trying to put together two combo boxes, Combo1 and Combo2, the content of Combo2 will depend on the selection made in Combo1, for example Combo1 will have 10 items, once an item is selected Combo2 will have different sub selections relating to that item. I know you can do this via Validation, but the problem with that I have about 4000 options in the combo1 and about 60000 in the second, here is the structure of my data:

First Option /Second Option
Option1_____1.1
Option1_____ 1.2
Option1_____ 1.3
Option2_____ 2.1
Option2_____ 2.2
Option2_____ 2.3
Option3_____ 3.1
Option3_____ 3.2
Option3_____ 3.3

My ultimate final result should look like this, two drop down menus once Option1 is selected second drop down form should display these options:

Option1 /_____ 1.1
____________ 1.2
____________ 1.3

see attachment for complete clarity