To convert an U16 array to an U8 Array of twice the length, just use typecast.
Wire the U16 array input on the left, and an empty U8 array to the "type" terminal on top. On the right, you'll get the new U8 Array.
(If the U16 array originates from the outside you also might need to know if it is little-endian or big-endian in order to know how to interpret the resulting U8 array.)