I understand you to be saying:
You have two rectangular tanks.
Each Tank is 34' x 10' x 11' maximum depth
Required Contact Time for Chlorination is 15 minutes
Required Contact Time for dechlorination is not stated in your original post
For Tank 1 (Chlor.)
Volume = 10'x34'x11' = 3740 cu ft (Maximum)
Volume = 7.48 gal/cu ft x 3740 cu ft = 27975 gal.
Flow Rate = 1000 gpm ( Maximum )
Available Contact time = 27975 gal/ 1000 gal/min = about 28 minutes
From this we can conclude that Tank 1 is large enough to provide the required contact time. Unless you wish to reduce the contact time to the minimum there seems to be no reason not to use Tank 1. You may want to install baffles in Tank 1 to ensure good mixing. Alternatively, because Tank 1 is larger than it needs to be to achieve the required contact time you could increase the flow rate, if you needed to.
Similar calculations may be done for Tank 2.
It is a good idea to define acronyms, such as SBR, the first time you use them. Even if most if your readers will know what they mean, some will not and you'll get fewer responses to your questions.
good luck