bidirectional synchronisation with one or more JIRA's

[Boris Georgiev _Appfire_]

As Nic said there are a lot of ways to do this - please provide some more details about what exactly do you want to sync. 

Here's one product by iDalko that allows syncing issues between instances

 http://www.exalate.com/ 

https://marketplace.atlassian.com/plugins/com.exalate.jiranode/server/overview

[Jobin Kuruvilla [Adaptavist]]

What exactly are you trying to synchronize? If it is issues and its fields, take a look at ConnectAll.

[Nic Brough -Adaptavist-]

There's lots of ways to do it, but I'd strongly recommend using add-ons.