Only if they are shot in a vacuum. When you factor in wind resistance, the 350 grain arrow will slow down much faster than the 600 grn arrow, simply because it doesn't have nearly the energy or momentum to fight thru the wind resistance like the 600 grn arrow does.

On the ballistics program, a 600 gn arrow and a 350 gn arrow, both at 300 fps: At 60 yards, the 600 gn arrow is still doing 283 fps and drops 75". The 350 gn arrow is doing 271 fps and drops 81".

That is assuming the arrows are shot flat, parallel with the ground and off a cliff.

Trajectory: assuming both arrows are sighted in at 60 yards. The 350 gn arrow would peak at 22" and be 20" low at 70 yards. The 600 gn arrow would peak at 20" and be 15" low at 70 yards. The 600 gn arrow would match the 350 gn arrow's trajectory only when it's speed is nocked down to 280 fps.

Of course, comparing a 600 gn arrow and a 350 gn arrow at the same speed is absurd, but it illustrates the point. You've got to consider wind resistance and how the reduced energy/momentum levels of lighter arrows inhibits their ability to cut through the air on the way to the target! And how much of their energy they use up just getting to the target.