yes, add a second dimple for the second set screw. Do it after you mock it up and line the gas tube up.
The reason you only have one is because the barrel maker can't know where the second one would be.
No standard exists, since the only actual standard is the A2 front sight/gas block assembly. It has only one hole, between the sling swivels, used as a clearance hole for drilling the gas port passage in the sight. Service rifle NM sights use that hole to retain the sights instead of the tapered pins and slots.
So a barrel maker will always know where the rearward dimple can be, opposite the gas port in the barrel.