Even identical twins will look different and have slightly different voices and body language so something would attract you to one over the other. But I'll assume hypothetically they were exactly identical in every way and saying the same phrases simultaneously so neither got your attention first while holding out identical bags at the same time. Perhaps you looked to the left first because you grew up in a culture reading from left to right. Perhaps the sun caught the metal on one of the bags and made it sparkle. Perhaps you're right handed and more likely to hold cash in your right hand which would put the bag on the right closer to you, so you gravitate towards that one. Or, most likely, you would hold out the cash and let whichever twin grabbed the money first make the decision for you.
Regardless, a lot more subconscious things go into this kind of "conscious choice" than people realize.