stratosphere.py

from troposphere import Template
import troposphere.ec2 as ec2
import stratosphere.ext
import stratosphere.cloudformation

t = Template()

# ...

t.add_resource(ec2.Instance(
    'ExampleInstance',
    ImageId=stratosphere.ext.LatestUbuntuImage(
        'trusty',
        Type='instance-store'
    ),
    SecurityGroups=[
        Ref(some_sg),
        stratosphere.cloudformation.Ref(
            StackName='different-stack',
            Resource='LogicalResourceId'
            # or ...
            Parameter='LogicalResourceId'
            # or ...
            Output='LogicalResourceId'
        )
    ]
))

# ...