Curl

You pose a logical question to which the answer is yes, the ribs always go one before regulating.

I suppose there are 2 reasons for this.

The first being that the sight bases go ontop of the ribs and as the sights are needed to regulate the rifle, the ribs of necessity must be present.

Secondly the chopper lump barrels are brazed at the lumps, you will see the excess brass spelter on the photos of the lump area in the pics posted by Mehul, the remainder of the barrels are free to the muzzles ( although it is usual for a barrel maker to make up some small ties from pieces of rolled up tin can which they use as spacers and solder them in between the two tubes as sort og gussets), the purpose of the ribs is both to hold the barrels together and also I suppose to keep them apart, the ribs impart the strength into the barrels.

Once the ribs are laid however the barrels become much more rigid but not necessarily inclined into the right place to regulate on the target, this is the painstaking bit.
The regulator must heat up the end of the barrel/rib area sufficient to permit movement to be applied to the end of the barrels by either adjusting the wedge (visible i nthe picture) or by raising or lowering one of the tubes to adjust for elevation of the resulting group.

Driving the wedge into the barrels forces the muzzles apart slightly and conversly drawing out the wedge will permit the muzzles to move together, heat is usually applied to this area in a conservative manner so as not to cause the entiore length of solder along the rib to melt. It is usual for the regulator to heat up two near bore size metal bars then place them into the muzzles, the heat transfer by conduction passes into the tubes sufficiently to melt the solder and permit the muzzle to be adjusted as required.

In reality movement of the barrel is effected by applying pressure and effectively bending the tube.

The process is a little hit or miss as it is often easy to over compensate with the movements required, this is the skilled part and one that can take some time to acihieve correctly, regulators who do a number of such rifles do get a feel for what adjustments are needed which can assist in speeding up the process.

I supose in the case of a .600 bore however it will be as much a case of the fortitude and ability of the regulator in absorbing the copious recoil from a number of rounds sufficient to see the job through that will be the biggest single headache