Multiple dominant follicle! What happn?

I have 2nos. Dominant follicle .. right ovary (18-19)mm and left ovary (20-21) mm I have couple of questions with respect to my scenario Q1. How will these rupture in natural scenario , if 21 mm follicle ruptures first what will happen to 19 mm follicle ? will both follicle rupture at once ? will only one follicle rupture at a time if so when will second follicle rupture ? Q 2. In IUI my doctor waits for natural follicle rupture instead of trigger shot ? I am confused that if I get follicular scanning on 19july Wednesday morning 10:30 am and on 20 th July 3 noon I get IUI done 29 HRS HAVE ALREADY PASSED ? and you are not sure when follicle ruptured inbetween ? Isn't the doctor fooling me and isn't it too late to do IUI and won't it be RIGHT TO GIVE TRIGGER SHOT AND KEEP CONTROL ON IUI TIMING ? Q3. Why IUI IS DONE POST FOLLICLE RUPTURE ON SCAN ? And under HCG TRIGGER SHOT U STILL GET 24-36 hrs before follicle ruptures ? Am I right ? But in natural rupture isn't it late