"I can't really tell"

I reply. Both women look at me first, then they look at each other, then back at me and say almost in unison:

"What do you mean?"

"Well" I start out "you are both really good cock suckers, but..."

Patti interrupts me: "But what?"

"But you have each just sucked my cock once, it is so close I cannot tell who is better. Right now you are neck and neck."

I can see that they understand what I am saying, but their competitiveness as sisters mean they want to get this resolved.

I decide to take a bit of a risk, so I say:

"We could do a cock sucking contest I guess."

Millie looks at me and says: "A what?"

"A cock sucking contest" Patti says, and continues: "I have heard of those."

Millie still looks confused, so she asks:

"What are the rules?"