You do realise that Kallis wasn't a world class batsman a great bowler together. He was a better bowler than batsman during the first half of his career and it was only when his bowling took a steep decline that he became a legendary batsman.
Same with Sanga. He is my favourite but if you compare him with say Dravid, Sanga has better stats than him but Dravid was the most consistent throughout his career. Sanga was ordinary to start with but he became one of the best batsmen ever since he gave up keeping gloves in Tests.
Honestly, it would be wrong if you think Sanga would be an excellent test match keeper and legendary batsman together because that never happened in real life. Kallis' career too had two facets: one when he was a better bowler but comparatively poor batsman and one where he was hardly an attacking bowler but a champion batsman.