You told them you're a wandering magician, and you're going on a one man tour around the country to show off some magic on the streets. Right now, you need a ride and would 'magic' away any heavy stuff (you included) in exchange for a free ride.

Obviously, the trucker Twins aren't buying it at first, because the way you worded your sentence makes them think you're trying to swindle them off their cargo. 

"What kind of magic you're offering anyway?" Danna asked you, with an eyebrow raised.

As a demonstration, you asked them to follow you to somewhere a bit private, at the back of their truck. There you pretended to say a few magic words, do a little hand gesture to sell the lie and finally pull the trigger of your Shrink-ray. In a flash of green you've eventually shrunken yourself. At the shock and amusement of the Trucker Twins.

"Look Danna Look!" Tanna shouted, jumping in glee. "They could shrink themselves!!" 

"Wow. Some magic you've got." Danna's eyes bulge out in shock, "Alright, shrink the cargo as promised and I'll see if we could squeeze you in."

Eventually you helped Danna and Tanna load a bunch of cargo, shrinking them down and loading them up. They were really thankful about it, especially since they had to go back and forth since the cargo they carry were quite big.

"Thanks to your magic, we can haul ass everything in a single turn. A miracle I say. How'd you do it?" Danna asked you, but you merely told them it was a magician's secret, so she simply shrugs it off. "Eh fine, seems complicated for fellas like us anyway."

Tanna approached you, "Say, about you hitchhiking, me and my twin thinks it's better if you're in front with us."

Danna went to her sister side next, "Cargo's quite full after all, and it doesn't have a good ventilation anyways. So, we figured one of us could carry you on the ride. Or wherever you wanted to seat, least what we can do as thanks."

You could do that, or is it safer to sit near the window instead? 

Rather, what if you sit between the twins, there's a small gap should the twin sits at each other. A gap you could slip in and be embraced by their thighs...