Can an 8″ rigid duct be split into two 6“ flex ducts

Basically you are trying to balance your new ducts with respect to your old ducts by sizing the new ducts. If you make a mistake or if your relative cooling loads (office vs rest of the house) ever changes, your only recourse would be to replace the ducts.

My preference is to size the ducts as large as reasonable possible (without increasing cost too much) and use a damper to tune the airflow ratios. Since you have 12" duct and a place to attach them, just get a damper like this:

http://www.grainger.com/Grainger/DAYTON-Round-Balancing-Damper-2TFY1?Pid=search

An uninsulated unshaded west wall in Texas can probably generate a very high cooling load in the afternoon.

Considering how much worse flexible duct is for airflow .vs. smoothwall, I think you'll be fine with 8"; better than it was, for certain.

Assuming real-world installation, this paper is quite enlightening (figure 2 also appears to show that 8" is slightly more than twice as good as 6" even under ideal conditions, where there's less difference between flex and smoothwall) but figure 3 is very enlightening, and that's for what's considered a "good, nominal" flex install.

Texas A&M airflow & pressure in flex ducts

Figures 4, 5 & 6 are downright scary.