Hello kenh,
I'd add my voice to the above comments.
But also draw attention to the difference in SDM versions. Even with the SDM "Multi-Site" feature available in SDM 12.6 and SDM 12.7, which allows joining of two separate SDM installations, I would recommend running on the same version of SDM. Unfortunately, the Multi-Site feature has been deprecated and no longer appears in SDM 12.9 or above, so I would NOT recommend it as a solution in this case - just drawing attention to an architectural concern.
I'd recommend speaking to a CA Services or Business Partner to further explain the business case. There are usually ins-and-outs that will influence the solution that will come out in a conversation. For example, with the information here, I may be tempted to recommend using BOXI Reporting to consolidate information from the two systems.
As Alex says, there are many integration possibilities, including using Web Services to create tickets in either system.
A single Service Desk Manager 12.9 with Advanced Availability and servers located in each geographic region may also be viable. There is a question mark over the access speed to the database from one location.
Thanks, Kyle_R.