Does a Second Rad Help at All?

[Nanook]

I want to run double rads in my case, one front and one top, but I can't help to think about the top rad not really helping much because it will just be getting preheated air from the front intake rad and board components.

 

Alternatively, I was thinking about just running one front intake rad but a very thick one instead (maybe 60mm) in a push pull  fan configuration.

 

Which setup would provide better cooling?

[Semper]

This will be entirely subjective to what you're trying to do. If a single 240mm/280mm is able to dissipate more heat than your system is producing, it will have no tangible benefit, unless you're looking to run much lower RPM fans (which also present other issues).
The pre-heated air factor that you're considering isn't much of a concern, it generally does result in *slightly* higher numbers, but it's nothing substantial or worrisome, unless you're pushing world records.

[Mick Naughty]

If you have the room, why not. 
I always run 2 rads at a minimum. 
Normally based on the hardware I run and the rad sizes, I don’t have much of a choice. 
 

Give it a shot and see.