The math works out as 18a + 50b = 36(a+b), where a and b are the ml of the 18mg and 50mg liquids, respectively.
18a + 50b = 36a + 36b ; 14b = 18a; b = 18/14a ; b = 9/7 a
You would use 9ml of 50 mg for each 7 ml of the 18mg. I always test the result, so let's see what happens with 7 ml and 9 ml:
18 mg/ml * 7 ml + 50 mg/ml * 9 ml = 126 + 450 = 576 mg total of nicotine.
576 mg/ (7 + 9 ml) = 576/16 = 36 mg/ml = yes, it ends up with the desired final strength.
That would be a really big dilution of the 18mg, over 50%, so it would likely harm the flavor a lot. As an early responder said, probably better to just pick up some 36mg/ml eliquid in a flavor that's "good enough" for craving points. Vapor4life (V4L), other places sell 36 mg/ml flavored eliquid.
Good luck!