Ngai and Enkai both mean God.
Historically and (mostly) pre-historically, Maasai and Kikuyu people have had a long and complicated history involving a lot of violent conflict that lead to a large amount of linguistic exchange between the two languages. I'm only familiar with Maa loans into Kikuyu but judging by the number of those I feel comfortable assuming some loanwords went the other way too.
The problem I have is with Ngai and Enkai. I've read here that Enkai means 'thunder, sky, god'. At first this lead me to immediately assume the word was loaned by the Kikuyu from the Maasai, but then I remembered that Kikuyu has the verb 'gaya', meaning divide (among), and one noun that can be derived from that is mũgai meaning "the one who divides/provides". Due to Kikuyu's gender system, you can derive that verb in a number of ways, including as Ngai.
- it could very easily have been Maa originating with Enkai and the Kikuyu borrowing it as Ngai, which is exactly how one would expect Maa words like that to be borrowed.
- or that it could have been originating as mũgai but, like many irregular verbs, switched gender from what is expected and ended up as Ngai, and then it was borrowed into Maa as Enkai.
- the two groups of people just happened to have converged on the exact same name for god, each with their own etymologies that show that it wasn't borrowed from another language.
personally i lean toward the conclusion that Enkai was just borrowed into Kikuyu which happened to have a similar sounding word with a similar meaning that it could be confused for. i have some other theories of a cracked pot nature, if anyone knows what exactly happened here i would be happy to learn more.