CBD and THC concentrates readily blend, once they've been extracted separately. If you have the GC analysis of the two, it should be as easy as ratio and proportion.
If you want to blend flowers and then extract them, to achieve your desired blend, you can start with a GC headspace analysis of the buds. You would also have to extract samples to determine the average oil content of the two strains, after which it would again be ratio and proportion, but with many more variables, making accuracy problematic.
If you just want high CBD and low THC, as JD notes there are strains meeting those requirements, specifically Hemp, where THC is below 0.3%.
Pure CBD crystals from Hemp are also available, and what I personally use in the topical that I make for my own pain relief.