This is definitely possible via a custom integration. We (Rego) have standard bi-direction integrations between PPM--SNOW, PPM-Jira, PPM-Rally, PPM--TFS. So, this is completely possible. You just need to decide on which system will be the master owner of what data, the triggers to move data and then the frequency to do it. The devil is always in the details for something like this but it is very possible.
Thanks
Josh