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Recently I had occasion to take notice that these two words are spelled differently. The difficulty is that they are pronounced exactly the same.....Right? I note that they share a common ancestor in the sense of "pleasing". However, to me the first connotes accepting current circumstances to one's potential peril, such as "Johnny was too complacent about the weather reports" and the latter to please by following rules. I am wondering also how we obtained this twofer.

When I look at the two words, I want to pronounce the first words with the second syllable accented. The second word being obviously French, I want to accent first and last syllable with nasality. Actually in reading them in context, I almost always pronounce them the same.

Complacent - pleased/confident in oneself.
Complaisant - pleasing/giving in to others.

Some time ago, they just split the difference. Why? Don't ask.

One rule about language I've come to accept is, "Never ask why." It's almost always a mystery. "Because" is the most common answer.