I tried it this weekend, and it went much better than I expected... You guys were all so right about everything, thanks so much for all the great advice!!
Anyway, the mare I was riding, the chestnut Arab, can be a bit of a bi-atch, but, just as you guys said, once I was riding, there was no biting, kicking or mean faces.
The only time the Arab got close to being rude was when we were trotting and the led mare would come too far forward alongside the Arab, and then she would make a little pissed off face (she is the dominant mare between the two of them, and I guess she liked being in front of the grey), but she never did anything.
So, in conclusion, I still have both my arms and both my horses, and it was super fun, so I'm definitely doing it again.
Here is a couple of pictures...
My view to the front:
My view to the rear:
By the way, is it fine/normal for the led horse to walk behind like this, or should I encourage her to come further forward? She mostly walked like this, or a little closer and to the side. When we trotted, she would be right next to us.