1: Every species and every era has a different transporter effect. Do we know why they are different?
2: When 2 transporter "pads"/stations are in play.
So 2 people go up and down with their fingers on the station - one on the from side and the second on to.
Is it safe? Is only one of them in control?
What if it is 2 different types of transporters? From a Federation ship to DS9, or like in ENT when V'Las beamed up to the Enterprise...