two barrels, cal. High Velocity .275 (7x57) on both barrels, product number 2492, barrel length 62 cm on both, on the first barrel two folding sights, 100 yards and 200 yards, the second barrel with fixed safari sight with the triangle of platinum, printed on both barrels: "John Rigby & Co.. 43 Sackville St. LONDON" - "sighted FOR Rigby SPECIAL .275" BORE HIGH VELOCITY CARTRIDGE Pointed BULLET 140 grain

It reads as if both barrels are sighted for the 140gn HV bullet, but if you look at the sighting arrangements I believe that is the diference that made them apropriate.
The first barrel with folding sights (and I think the two folding sights is wrong as onefor 100yds would be fixed with the second and folding blade for the 200yd range) would be more suited to open ground hunting while he may have wanted to use the second barrel with the fixed safari sight highlighted by the triangle of platinum for dark days, heavy cover hunting, driven game where speed of sight aquisition is important, and for the odd foray into Africa where it was possibly intended for the bushveld.

Dated from 1905 when qd scope wernt necessarily reliable nor apropriate and I can indeed see the logic in two barrels in the same caliber especially as they are in 7x57.

Von Gruff.