So the pastry needs to be two half-baked pie shells. Grab any two crust pie recipe, put in two pie shells, prick each shell a half dozen times on the bottom, and bake at 400F for 20-25 minutes, until the crust is browned. Some people like to use parchment and pie weights, but I think that's unnecessary as long as you check ten minutes in to see if the bottom is swelling, and, if so, you just poke it to make it deflate, and then there's no need for parchment or weights. You can pre-bake these while prepping the filling.
Before baking the pies though, prep the fruit. Peel and core your apples, chop them up and toss them with 1/4 cup of the sugar, then chop the rhubarb and toss it with the other 1/4 cup of sugar, then set them aside (covered) to macerate. The apples need to sit for at least half an hour before you use them in the next step. Once your pie shells are in the oven, whisk well the cream, flour, and eggs. Add the chopped apples plus their juices and sugar to the custard mixture, and zap it with an immersion blender, stand blender, or food processor, whatever gets the job done. The apple should be soft by this point and should just blend away without leaving any chunks in the custard.
Now comes a quick choice for you to make. Basically, I wanted the custard to be strictly apple flavoured, butyou might like the idea of mixing the flavours. So, if you want your filling to taste of both apple and rhubarb, toss the chopped rhubarb plus it's juice into the custard mixture and pour it all into the shells. If, like me, you want segregation of you flavour states, pour the custard into the shells pour off and reserve your rhubarb juice (I made mango-rhubarb lassi with it!), and toss the rhubarb over top of the custard.
Bake in a 350F oven for 25-30 minutes, until the custard has set.
So, as this is from memory, there are caveats. I have no idea how much rhubarb I used, just use what you want and what you have on hand. I scarcely buy it and instead just get bundles of it from people who have it and have too much (rhubarb is like a weed). As well, I might have used four eggs, I might have used three. Feel free to use as many eggs as you want if you have a better idea.
It fulfilled my objectives pretty much to the letter, by having a very apply custard plus chopped rhubarb. The custard was a bit soft/watery, though, so it might be a thought to either not add the juices from the apples or to reduce that juice first. I was fine with it as it was, but you might have a different opinion after making.