Check train routes and pricing here:
https://www.trenitalia.com/content/tcom/en.html
Just use same day of week to check now as Sept schedules won't be up til June- schedules don't change much
Station names:
Milano Centrale
Stresa
Firenze SM Novella
MIlan to Florence via fast train is about 2 hours
Milan to Stresa is about 1 hour- regional train
If you fly in to Florence you still need to get into center to train station then fast train to Milan- then regional to Stresa << this would be my last choice- too much travel esp if coming from US
if you fly in to Milan- there is a direct bus to Stresa from MXP
After wedding- regional to Milan from Stresa then fast train to Florence
If possible fly into one and out of the other. That will save you a whole lot of time and expense and trains, etc
You need to be in your departure city night before flight- (Stresa is fine as night before departure because it has direct bus to MXP and is actually closer than Milan).