It’s a matter of preference. If the dimpled screw doesn’t budge then dimpling the other gains you nothing.
The single dimple give you a reference point for perfect alignment (assuming they did the dimple exactly 180* from the gas port). Plus if you use a gas block that has non-standard screw spacing, now you can do the second dimple in the correct location.
For the extra pieces of mind, I like dimpling both. Set the first one, remove the second screw, dimple the second hole using the gas block as your guide, install screw, loctite/rockset if preferred, done.