I've adopted Atlas a blue haired person and I see pictures of families with two rare people. Is there a technique to getting rare people to propose or is it just luck?

just luck

Since one of my families reached the 100 Trophies a few months ago, I've been using Lyssaria's method of trying to get the proposal that I want. After starting a new generation I wait about 1 minute, pause and close the game and save a copy of the game file for that family. So, if I just started a new generation in the 4th game slot I:
1. Using the procedure for backing up game files for Windows or Mac to access the files for copying, I make a copy of "Virtual Families4.ldw" and paste it on my computer desktop. I make sure the copy is named "Virtual Families4.ldw" and not "Virtual Families4.ldw copy".

2. I then delete the original game file from the folder and paste in the copy I made, then restart the game. A proposal should come momentarily. If it's not a suitable mate then I reject it, reopen the game folder, toss out the file and repaste the one from my desktop. I do this over and over again.

3. You're not guaranteed the exact proposal you want but sometimes you can get pretty close.

It all really depends on how much patience you have and how long you're willing to go around and around looking for the ideal proposal.

I spent 2 hours (at least 100 proposals) and finally decided to accept the 3rd head from the bottom on the head chart (which I had also saved along the way).