Couple of questions about the Minotaur Double Crossbow:
1. "Due to its size and weight, you take a –2 penalty on your attack roll if you’re proficient"
Does that mean that a small creature is using a medium crossbow (therefore dealing 1d8 instead of 1d6) and a medium creature is using a large crossbow (dealing 2d6 instead of 1d8)?
2. Let's say bolts damage is 1d8+10. Would total damage be 2d8+20 or 2d8+10
1) no, you take the -2 to hit for a weapon sized for you. (I doubt that you can but) IF you could wield an oversized double crossbow you would take the size penalty as well.
2) Depends on what the +10 is made up of. Most damage will double up. The key part is: "Critical hits, sneak attack damage, and other precision-based damage apply to only the first bolt."