Do I need to run 2 instances on bitcoind / bitcoin-qt.exe for that?

Yes. If bitcoin-qt is already running and on mainnet then open up a command shell and type:

>bitcoin-qt -testnet

This will launch a second bitcoin-qt on testnet using the default -datadir= and -conf= options.

The -testnet option uses the folder {default bitcoin datadir path}/testnet3/ and the default bitcoin.conf file by default. If you wish for your testnet instance to have different options to your mainnet instance then add the -conf={file} option after -testnet on the command line.

This works much the same for bitcoind.