I am uncertain of rift lake with CA/ SA mainly due to the PH water parameters required. Also "rift lake" is extremely broad. Another poster made mention of "rift lake" water ph is around 8. That is TOTALLY false and NOT AT ALL a good idea.
While I can accept that a PH kept steady in the 7-8 range MAY work for CA/ SA and Victorian Rift lake or possibly even Malawi rift lake cichlids, but the rift lake "Tanganyka" IS NOT EVEN within the 7-8 PH range.
The fact that (A FISHKEEPER) <== Not pointing fingers at you, is able to keep rift cichlids alive in a tank of fish totally alien to its natural habitants and in a set of water parameters severley detrimental to it's husbandry only guarantees that SOME, if not ALL of your fish are LESS than 100% comfortable in these setup(s).
As long as the fishkeeper is unconcerned with supplying their aquatic pets with the best possible living conditions, then the different PH requirement is a total non-issue.
You asked, you got the answer. PH!!!!!
Please DO NOT mis-interpret Otherone's comment on the steady 7-8 PH range is "ideal" for both types of cichlids, only that it can produce an environment where both can survive. Otherone, I am sorry if it is ME mis-interpreting your statment, and if you are indeed infering that it is an IDEAL than you are flat out wrong.
A fish that requires 7.0 - 7.2 PH as "ideal" is to some degree suffering from a PH of 7.5 just the same as a fish that requires ideal PH between 7.8 - 8.0 is suffering. This compromise is really nthing more that "misery loves company".
Why stop at not caring about PH in the water parameters this be the case? Why not stick brackish fish in your marine tanks with slightly lower salt content. The low end Marine POSSIBLY could survive in a high end brackish.
Hell, wouldn't Scats, Archers and Dragon Gobies be awesome in the same tank as Severums, Datnoids and Bichirs?