I am getting ready to try DIY also, bought the VG and PG already, need to order my flavors after the holiday season.
From what I understand reading here and many other posts on the forums is that you basically want to break your flavorings into 2 groups to start out.
Primary Flavors - These are your base flavors, so the flavors that you want to be "strong" in your mix, so in your case both (Coconut + FV) I believe.
Secondary Flavors - These are your enhancer flavors, creams, sweeteners, or accent flavors.
So in this case if you liked FV at 20%, and Coconut at 10% in standalone mixtures, then in a combined mixture you would cut them in half. However then you will want to find which ratio they work together best at, and keep that ratio as you move your total primary flavor percentages up or down depending on tastes.
If you mixed up that recipe I am guessing the flavor would be way too bold, and you would spend a lot of time notching it back before you hit 50% of original.
If you want to have a rough idea of what they will taste like at the reduced flavors, mix a small batch of your two standalones together in equal parts and let it sit for a bit to mix together. That would effectively give you a liquid with 5% coconut and 10% FV.
Hope I could help, I am still new to this as well, but trying to do lots of research before I get started.