does two x 99% sure make 198% ?

I don't think you can do anything about that. Medieval II seems to have some setting you can adjust (relating to age) but RTW doesn't.

For frequency of childbirth, you can have traits with
Level Fruitful
Description Fruitful_desc
EffectsDescription Fruitful_effects_desc
Threshold 1

Effect Fertility 1
which should in theory increase that particular generals chance of having kid, but that's about it and I'm not sure how well that works.