Behat Minkextension multiple capabilities for sending test to BrowserStack

I am trying to send my test to multiple browsers on BrowserStack. For example Firefox and Chrome on Mac.

extensions:
  Behat\MinkExtension:
    base_url: [url]
    sessions:
      my_session:
        browser_stack:
            username: "username"
            access_key: "accesskey"
              capabilities:
              - {"name":"domainname","browser" :"Chrome", "os": "OS X", "os_version": "Yosemite", "browserstack-debug": true}
              - {"name":"domainname","browser" :"Firefox", "os": "OS X", "os_version": "Yosemite", "browserstack-debug": true}

I also tried to add them as this:

[ {"name":"[domainname]","browser" :"Chrome", "os": "OS X", "os_version": "Yosemite", "browserstack-debug": true},  {"name":"[domainname]","browser" :"Chrome", "os": "OS X", "os_version": "Yosemite", "browserstack-debug": true}]

But this doesn't work either. This are the dependencies that I installed:

    "behat/mink": "*",
    "behat/behat": "*",
    "behat/mink-goutte-driver": "*",
    "behat/mink-selenium2-driver": "*",
    "behat/mink-extension": "*",
    "browserstack/browserstack-local": "dev-master",
    "ciaranmcnulty/behat-stepthroughextension": "dev-master"

The browserstack/browserstack-local is only supporting Behat version 2.5 and I'm using 3.4.1.

I hope you know an answer :)