What should I do with 40 egg yolks and 2 pounds of black tahini?

For the egg yolks, cure them. Then you can use like grated parmesan cheese. Here's a recipe: http://www.bonappetit.com/recipe/cured-egg-yolks

Along the same vein, make soy sauce-pickled eggs. http://cooking.nytimes.com/recipes/1017850-soy-sauce-pickled-eggs

These are 2 projects at the top of my to-make list :)

I would treat them separately. Simply because I would make egg yolk pasta, which is my idea of heaven. Ruth Reichl gives a recipe for 18 yolk noodles in her "Comfort Me With Apples". Tahini will last for a long time, so you can use it for a variety of things.

I would make a tahini fudge (a 'variant' of the classic Til gul candy from Western India). https://food52.com/recipes/41942-sesame-tahini-fudge

You don't say what format (or state, as we used to say in science class) the egg yolks are found in. Solid or liquid?
For either or both, freeze in usable quantities and decide later.
From solid (i.e., hard cooked) make mayonnaise.
From liquid, make custards and/or ice cream.
From liquid, make lemon curd...and give as gifts. Worth its weight in gold.
From both the egg yolks and the black tahini, make some exotic ice cream (have no idea if that will work, just a possible idea).