An Arm and a Leg
The phrase "an arm and leg" usually refers to the fact that something is expensive. It is also a literal meaning: to acquire the said thing that is expensive you must offer up an actual arm and leg.
Which Arm and Which Leg?
This is a great question and has been the subject of debate for millennia. The phrase doesn't specify which arm and which leg. In fact, it's up to debate if it actually refers to one's own arm and leg. Could it refer to someone else's arm and leg? Possibly. What is certain, though, is that If something costs "an arm and a leg" presumably it considers one's dominant side as more valuable. Thus, a right-handed person would be expected to offer up their right arm and so forth.
Can I Offer Two Arms or Two Legs Instead?
No.