Unless you have a lot of time, I certainly would not advise Kenya and South Africa on the same trip. And I would not attempt Kenya wildlife without a tour operator. You might be able to find a decent tour operator with custom tented camping, better IMO than staying at a lodge except for two two-hour van runs following "the track" to look at surrounded wildlife.
I went on such a safari with Safaris Unlimited when it was owned by Tony Church, and it is now operated by his son Gordon. Longtime company, extremely knowledgeable Kenya hands, offer luxury tented and horseback safaris. They may also be able to recommend something if they can not offer to very small groups.
www.safarisunlimited.com
I'd also not recommend consider diving in (or near) Mombasa, though there are some OK places north of Mombasa, such as Watamu.
Or, fix your eyes on South Africa and visit there (and maybe Botswana). I like tented safaris with Penduka Safaris for a wilderness experience, for example:
www.pendukasafaris.com
Check Tanzania out:
http://www.thomsonsafaris.com. Rick and Sally have been "old Tanzania hands" since the 1980s, provide smaller tented experiences, etc.
Best: do one now, do another later.