Professional first and second serves have a very different direction of effort.
The first serve has more of a horizontal effort than the second, which is hit upwards almost vertically.
On grass, at Wimbledon, pro players prefer a slice serve over the topspin serve, making the difference between the two serves less appreciable, but they are still distinguishable.
Be mindful that top players first serves still have about 3,000 RPM rotation, while seconds serves can be in excess of 5,000 RPM.
Air resistance affects ball speed. A serve normally loses more than half the speed from baseline to baseline, less with the skid factor on grass.
Unless you play on grass, you should make your second serve very different from the serve. While you can put the same energy and force on the second, most of this force should create rotation, so as to assure a higher net clearance and a steeper dropping curve.
Hit up as if you were reaching for the sky, brush the ball up in excess, and you’ll lose the fear of hitting the second serve too hard.*
*For a complete description of this technique check out my DVD Tennis Into the Future: Modern Serves and Volleys.