This cannot be done. A TN cluster is formed by:
- Sharing the same DB tables
- Optionally, configuring the TN instances to notify other instances when they make profile, doc type, rules, etc. changes. The other instances respond by reloading their TN caches.
The DB tables are generally version specific. The TN tables between 6.1 and 7.1.2 differ. Indeed, 6.1 and 6.5 differ. So different TN versions cannot be put together in a TN cluster.
Additionally, the TN docs state that the TN export/import facilities are not intended to be used to export TN objects from one version and imported to another version.
If you want to migrate the 6.1 data, follow the migration docs to create a copy of the 6.1 tables and convert those to 7.1.2.