What code did you use to make the main account? If it was the same type of code that you're trying to apply then it won't let you. Same as CoH + CoH. I assume its to stop you paying for time with cheap games. 
If main account was made with CoV and you're trying to add a CoV Deluxe/Collectors I 'think' that works but couldn't swear to it.
My Main account is CoH Deluxe + CoV Collectors + GvE and I am now trying to apply a second CoV (original) code.
Oh well - I can give the code to a buddy and hope he will reactivate his basic CoH account. 
Uh a collector like me, and a from the same country no less!
So far I got:
CoH Retail
CoH Deluxe
CoV Ore-Order
CoV Retail
CoV Collector's Edition
Good Vs. Evil
And the downloadable (I want a shiny box!):
Wedding Pack
Cyborg Pack
I would've loooooved to get a pre-order CoH, but
unfortunately I wasn't hooked enough when the EU pre-order
came out... 
Ah well... guess you can't have everything. 
I was under the impression that you add several CoV serial codes of the same type to the same account.
So I purchased standard CoV box from Amazon, but when I select upgrade on my main account and enter the code, I only get the choice to upgrade my CoH trial accounts to a full account.
How can I apply the code to my main account?