Hi there,
I could never put this to work... I have used a workaround as follows:
1. Use a link instead of a button
2. On the link, add an extended property, with name "target" and value "_blank". This will open the destination on a different window. It doesn't work on a button. To make your link look like a button, take a look at: https://stackoverflow.com/questions/17207342/input-button-target-blank-wont-work 
3. Change your code, so that what you need to do happens in the Preparation action of the destination, instead of the action fired when you click the button.

Alternative:
1. Instead of redirecting to your final page, redirect to an intermediate page DummyPage
2. On DummyPage, put a link to your final page, with the extended property target=_blank
3. On DummyPage, put some javascript to simulate the click on the link as soon as you enter the page

Maybe this alternative won't work, because the original tab would actually navigate to DummyPage. You would have to try it out.

Let me know if this helps.
Best regards,
Pedro Vieira