First let see if we can do it in a utopic world:
1. Client with a 10Gig card, should be able to move up to 1200MB/sec (Utopia)
2. DataDomain box with 2x10Gig Cart, should be able to move up to 2400MB/sec (Utopia)
3. If the DD is doing nothing and just dedicated for this client the math says:
12TB~12582912MB
So our limit is 1200MB/sec which is the Client 10Gig card speed limit, is a 10hour window so our limit is:
10hr*60min*60sec=36000secs
1200MB/sec*36,000secs=43200000MB~41TB
So by math if the DD box is dedicated or at least one of the 10Gig ports is fully dedicated for this backup it can be done without a problem, but if go in to reality lets say we can perform only at 50%, still is 20TB in 10 hours, so you should be fine with your jobs running at Quater of the speed, meanin other jobs can run at the same time and you should be able to achive your backup window, of course all the tuning needs to be done, talking about buffers, nic, tcp/ip tuning and key piece Oracle DB response if they DB cannot follow the pace of the backup speed it will be your first bottleneck.
Hope this helps.
Regards.