Can 2 - 6" flex ducts be combined into a single 8" rigid duct
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&amp;M airflow &amp; pressure in flex ducts

Figures 4, 5 &amp; 6 are downright scary.