Is the 8 x 68 and the 8 x 68S the same cartridge? I just looked up loads for the 8 x 68S and it looks to me like a 35 Whelen with 225 Gr Bullets at 2800fps has it beat. Just slightly mind you, but anything you can take with a 6 x 68S you can take with the 35 Whelen. So it too, could be a one rifle battery.
Same thing.
The 8 x 68S will beat the 35 Whelen in the field as it shoots a slightly smaller diameter bullet with a higher SD and BC in the same weight at higher velocity. The 8 x 68S does its best work with heavier bullets at long range, a superb African cartridge for the job on the toughest African plains game.
Typical factory specs:
35 Whelen
225 gr (15 g) SBT 2,613 ft/s (796 m/s) 3,412 ft·lbf (4,626 J)
8 x 68S
14.5 g (224 gr) RWS KS 870 m/s (2,900 ft/s) 5,488 J (4,048 ft·lbf)
Both are excellent cartridges and good choices as one rifle battery, for larger game out to longer range, and as you mention they can take the same animals, but I would personally give the 8 x 68S the nod ahead of the 35 Whelen.