The attribute modifiers for both included attributes apply, added together :) But remember! Tiered skills have a higher TN than their simpler counterparts! (though it sounds like you knew this already)
In example, Dex 4, is no modifer, but Intelligence 8, awards you a +1! Add that right onto your skill modifier (which is at least a 4, for being Computers/Advanced), and that additionally plus any other bonuses is in your favor.
If your Dex was a 7 and Int still an 8, you would be given +1 for extreme Dexterity, and +1 for extreme intelligence, combining for a +2 Linked Attribute bonus! This applies the same way as it does for all the other dual linked attribute skills.