Hi David,
Are there any issues with the Server running both versions at the same time?
If you are careful about the database server and database client configuration, no, there should not be issues with this arrangement. Be aware of server hardware resourcing limits when you run multiple database servers on the same server.
Also be aware that whenever you have a database server starting up, it should be uniquely named (dbsrv12 -n server option), and every database you are connecting to is also uniquely named on that database server (-n database option). Then in your connection string for your application, be sure to specify the associated "SERVER=" and "DBN=" connection parameters with your database that you specified in the server configuration.
If you are seeing specific issues when running these servers consecutively, please reply back and we can try to help you out with your specific issue.
Regards,
Jeff Albion
SAP Active Global Support