So I threw away my WC3 cds long ago and now I want to start playing again.

Here is my question: can you run frozen throne on battlenet if your RoC cdkey is the same as someone else's?

yes

The problem with this is if two users are using the same RoC CDKey and a different TFT Cd Key they both cannot sign on the same server at the same time. It will say"This CD Key is in use by USERNAME"

So while you can use your TFT cdkey to DL the game from battle.net, you cannot play on the same server as the other person sharing the RoC cdkey with you even if you are playing TFT.

I hope this helps, pm me if you have any questions about this.

Ah, damn. Thanks for the clarification ^^