Hi,
Use connector to exchange demographic data between the platforms.
Use iframes to present activity data from one platform in the other.
If you do have a need to exchange activity data, consider converting it into demographic data via aggregation- e.g. instead of sending all purchases in order to find out if a recipient's purchased something in the last 60 days, create a flag called 'has purchased something in the last 60 days'.
Thanks,
-Jon