Now that monotypes are essentially done (well, besides Dragon I guess), I decided to try a dual type run where both types favour the same area.
For this run, I used 2 types that boosted the same area to the same rate. In this case, I used Joltik and Galvantula. Bug and Electric favour Forest and both boost it to about 50%. These are the results:
Forest is about 50%. By the looks of it, the rate doesn't change if the rates of the individual types are the same. Unfortunately, this will probably mean that Fire/Fighting types will not be any different from pure Fire and pure Fighting types, so the best rate you can have for Mountain is 50%. Oh well.
Next time I'll test a dual type where the area favoured is the same but the rate is different.