I am sure someone has posted this before but, I am new so that is my excuse :)
Do not jump on the back of a bike just because the guy is nice,
Ask a few questions
How long have you been riding?
How many miles have you riden on this particular motorcycle?
How often do you ride two up?
How many miles have you riden two up?
Sure there are others but, you get the idea. Then you should expect him to ask you some questions.
Have you ever riden?
Have you ever ridden two up?
How far did you ride two up?
Did the person talk to you about riding two up? in other words did they explain what they expect from you as a passenger
If you have never ridden the driver should explain alot of things to you being a passenger on a MC is totally different from a cage.

Good advice I hope the girls listen to you!!!

Ride safe