Communication issue between two different Citect versions

Mark Gu  Why the two different versions of Citect?
Do you have a PLC that talks to both systems, I would put this stuff in the PLC but then again you now have a middle man that is a single source of failure.

Because the two systems were configured different time, the v8.2 system is upgraded from the old one. but actually we also have a separate target PC with v7.5 also has such communication issue, same version.

I have a question along similar lines. 
We have a power station that runs Citect 2018 R2.  On the same site is a processing plant owned & run by a different company, which happens to also have Citect SCADA.  I'm in the process of finding out which version they use. 
They have asked to see some of our station data, and making them a small project that they can "Include" in their Citect system seems like a neat way of doing it. 

Is there a Compatibility Matrix for different Citect versions being able to share variable tags? 
I'm assuming in this case that the other party is unlikely to have exactly the same version of Citect as ours, so I'd like to know whether a Citect-to-Citect link will work once I know what the other version is.   

Not seen or done a Citect to Citect Link, Would avoid it anyway. Maybe now its possible with OPC UA  Server-Client but will probably have to upgrade to a later version (2023), haven't tested this.  The best way in my opinion is if both Citect systems can see where this data is coming from then they can independently get the data depending on your field device supporting multiple connections to it.

I am lost on your architecture, is this a primary standby setup with two different version? or two independent systems on different version, curious exactly how they would be talking in this case.

Thanks.  In this case, multiple Citect Servers is not an option unfortunately, as a number of the Modbus TCP Server devices allow only a single Modbus TCP Client to poll them.  
I have subsequently discovered that the other version involved is Plant SCADA 2023. 
I agree that having 2023 on both ends would likely resolve this, but we have a collection of sites running 2018 R2 using common templates and so on, so our preference is to keep them consistent if possible. 
I'll check directly with Aveva Support and post back to close this thread if I get a definitive answer. 

I am really confused. Are you using disk I/O and pointing to the same disk I/O file saved somewhere both servers can access? That will not work.

Best bet would be to configure an OPC DA server on both servers and use some 3rd party OPC middleware to do the data exchange. Or an OPC tunneller.

Alternatively I'd just take SCADA out of the equation completely if you can and stick a protocol gateway/converter in there so your PLCs can communicate directly with each other.