External support ticket sharing

One of our vendors has followed our example and moved to using Zoho Support for thier Help Desk. (Any kickbacks?)

Is there a way to add them into our Support Desk to allow them access to a particular department so that we can share tickets and assign them certain tickets or tasks?

We could of course simply email their system, however if we do it from within Zoho, then our ticket numbers in the subject conflict and either causes an auto-response loop based on our mutual reply settings, or wrong ticket numbers are updated.

Not sure if anyone else has run into this. Of course if we could modify the subject line ticket numbers this wouldn't be such an issue. For example we could use [##ME_ticketnumber##] and our vendor could use something unique to them. This would also prevent loops from developing with other automated response systems.

Thank you,