user75737: How to run Bitcoin Core both main and testnet on the same machine?
I want to be able to communicate with both block-chains via RPC.
Do I need to run 2 instances on bitcoind / bitcoin-qt.exe for that?

user75001: 
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.

75737: <p>How to run Bitcoin Core both main and testnet on the same machine?
I want to be able to communicate with both block-chains via RPC.</p>

<p>Do I need to run 2 instances on bitcoind / bitcoin-qt.exe for that?</p>
 75001: <blockquote>
  <p>Do I need to run 2 instances on bitcoind / bitcoin-qt.exe for that?</p>
</blockquote>

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

<pre><code>&gt;bitcoin-qt -testnet
</code></pre>

<p>This will launch a second bitcoin-qt on testnet using the default <code>-datadir=</code> and <code>-conf=</code> options.</p>

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

<p>This works much the same for <code>bitcoind</code>.</p>
