What
Red Leg is discussing is a little different, but well known by the old hands...
In effect a double rifle is two rifles attached together, right? Two barrels, two firing groups, etc.
Therefore, one can use a double rifle as a double rifle ... as a single shot rifle.
The process is simple: with a scoped double rifle, one sights one barrel (ONE barrel only - generally the right (first) barrel) like one would sight a bolt rifle. What Red Leg says is that each barrel on his S2 is sub MOA, which means that if he fires 3 shots in a row from his right barrel, he gets a 1" group at 100 yd. The 500/416 being indeed in essence a rimmed .416 Rigby, i.e. it has a near 200 yard MPBR, this means that his S2 is shooting day in, day out, 2" groups at 200 yards with that one barrel. This is how one uses a double rifle as a longer range single shot rifle.
So,
Red Leg's S2 continues to be a typical double rifle, grouping its TWO barrels 2.5" out to 100 yards, but his right barrel INDIVIDUALLY shoots 1" groups at 100 yards, therefore the rifle in appropriate longer range caliber - which the 500/416 is - is perfectly capable of taking a Kudu at 200 yards when it is scoped.
By the way this is not unique to S2 rifles. Any scoped double rifle can be used that way, and just like bolt rifles, some double rifles have more or less accurate barrels. In general, because they are very rigid (being soldered to another barrel), each individual barrel tend to be pretty accurate (if the crown has not be messed with to try to make the barrels shoot together).
It is actually how scoped British .375 double barrel rifles have been used by folks in the know traveling the world (India/Africa/America) with only one rifle, since the availability of scopes reliable and compact enough to be mounted on doubles. The point being that a .375 double, or now a 500/416 double, gives you truly "three" rifles in one: a hard hitting DG double rifle, and a flat shooting single shot PG rifle.
Does this help?