Yes, you can copy selected thoughts from one brain to another. To do this, you would either CTRL+Click (CMD+Click on a Mac) on the thoughts that you want to copy and move, CTRL+Click on any of the 'gates' surrounding a thought to copy all thoughts below, above or next to the thought or click on EDIT>"Crawl Brain and Modify Selection..." and select your options accordingly. These methods will put your selected thoughts into the selection box on the left side of your plex. You would then Right-Click anywhere in the selection box and choose "Copy Selected Thoughts". Lastly, you will want to go to your new brain, Right-Click the plex and choose "Paste Thoughts".
You can also follow this link below for better understanding regarding these methods: (http://www.thebrain.com/support/tutorials/multi-brains/copy-and-merge/).