Replace a triangle with the letter $ a $ and a circle with the letter $ b $. The two given equations become \begin{align*} 5a+3b&=21\\ 3a+5b&=16 \end{align*}Multiplying the first equation by $ 3, $ we get $ 15a+9b=63 $. Multiplying the second equation by $ 5, $ we get $ 15a+25b=80 $. Subtracting this last equation from the second-to-last equation to eliminate $ a, $ we have $ 16b=17 $. Multiplying both sides by $ \frac{2}{16}, $ we get $$\frac{2}{16}\cdot 16b = \frac{2}{16} \cdot 17 \implies 2b=\frac{17}{8}.$$Thus, two circles equals $ \boxed{\frac{17}{8}} $.