Hi,
if I use 2 x 8mm reinforcement bars compared to 1 x 16mm.
Simply put:
just calculate the cross-sectional areas, that should answer your question:
Cross-section of the 8mm reinforcement: 50 mm^2 (4*4*pi)
Cross-section of the 16mm reinforcement: 200 mm^2 (8*8*pi)
The tensile strength of reinforced concrete is largely determined by the reinforcement. Replacing one 16mm bar with two 8mm bars is therefore not feasible; you would rather need 4 8mm bars.
In reality, it is probably not that simple, as there will be other influencing factors, such as the distribution and position of the reinforcement.
Regards,
Andreas