Is there any advantage in keeping dual video cards? I just upgraded my graphics card from Nvidia GTX 1660-Ti to Nvidia RTX 2070 SUPER. 

Is there any advantage in leaving the 1660-Ti in? The 2070 has shown a 30% increase in benchmark so far!
Yes leave it! You can render with both cards, and the CPU too, all at once. Just make sure you go into Blender parameters and turn them on under Cycles render parameters.