One of the biggest challenges to learning Dynamics Marketing last year was simply getting your hands on it. That changed with the recent release now you can get a 30-day trial just like CRM. You can sign up for a trial here - Signup for 30 day Marketing trial here
If you wish to have CRM and Marketing trials together, you can use one of the following options
Start a new Marketing Trial, and then go to Purchases Services in the Office 365 portal, locate Dynamics CRM and click Trial
Start a new CRM Trial, then go to Sign up for 30 day Marketing trial here while logged into the CRM trial account and it will ask if you want to add it to the tenant. Marketing currently doesn't show a Trial link in the Office 365 Portal under Purchase Services that's why you need to do this a little differently.