Nope.
For example, if you have a city producing 60 shields, then you can NOT produce two 30-shield units in one turn.
Excess shield production is not accumulated.
Enjoy your learning experience!
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JB
I play BtS (3.19) -- Noble or Prince, Rome, marathon speed, huge hemispheres (2 of them), aggressive AI, no tech brokering. I enjoy the Hephmod Beyond mod. For all non-civ computer uses, including internet, I use a Mac.