Kizaru is stronger then both of them individually, I think it was made clear during the war that first mates are not on the C3 admirals level, they were below them, seeing how WB(WSM) itself struggled to do significant damage to any admirals until he got blood lusted and got a free hit in akainu, marcos haki was considered as an irritation to akainu, and somehow he was able to fend off Marco,vista + other commanders by himself even after getting the beating of his life, then again ppl may say that Akainu is just above the other admirals, but its clear cut that the C3's strength gap is so little that its hardly worth a mention , seeing how aokiji was able to fight akainu for 10 days straight, for me thats worse than extreme diff, and I dont think kizaru falls of from that category so somehow:
Akainu-10 days>Aokiji-10 days>Kizaru