accroding to the demo pneumatics manual, the air tanks are not hooked together in series, but rather where two combined with a T joint, is there an advatnage to this design rather than just hooking in series with two tanks and putting the T joint later? 
In terms of pressure and/or efficiency, probably not. Having them in parallel just means it takes up less space length wise.
the way i see it is its like pouring from two soda bottles at one time compared to pouring from a larger bottle, and clearly the first case would work faster
The increase isn’t significant (or at least from what i’ve experienced). The size of the pipe still limits the air flow, even if you put them in series. The only way to achieve higher air flow is to increase the PSI (the max is 120 anyways) or get bigger pipes.
If you had bigger pipes, then connecting the tanks in series would definetaly increase the air flow rate, since the bottleneck is now at the accumulator tanks.
For example, with your soda bottle example, try connecing them to a hose the size of the bottle’s neck and empty them out, there won’t be much of a difference if you had one big bottle, or two small ones connected in series.