Glad to see you already solve your problem, but why would you need to run 2 instance of Bitcoin Core at same time?
I don't see any benefit from it and you need extra costs to store 2 copy of exactly same files (unless you run mainnet and testnet node).
You don't need to do anything in terms of custom args (-datadir=C:) to run testnet, when you run the custom binaries it automatically separates the testnet blockchain on it's own folder.
Honestly the only use ever I did from from -datadir=C: was if im not mistaken, when I wanted to access a fork of Bitcoin, because idiot devs decided to overwrite Core's with their client. Probably must have been Vercoin or some shit along the lines.