So... Kaladesh Prerelease. I'm wanting to purchase two prerelease packs. My question ISN'T: "Can I fuse boxes and have more rares than anyone else and dominate?" My question is, can I open both boxes at the event and choose which card pool I would like to use? Or can you only use one predetermined box and open the other later? Are there even rules regarding this?
You cannot bring extra prerelease packs into the tournament like that.
Limited tournaments (such as drafts) work like this: The tournament officials (such as the game store owners running the pre-release) issue you boosters/cards, and they must issue everyone the exact same amount of cards. They will issue you one pre-release box, because if they issue you two, they have to issue everyone else two as well. That is the one and only box you get to use.
If you happen to possess other boosters or pre-release boxes, they don't count (even if you bought them from the tournament organisers, since they weren't issued to you for use in the tournament). They're outside the tournament scope, and you can't bring them into the tournament and use the cards inside them for that tournament, even to pick between them and another box. You use exclusively the box and boosters given for the tournament entry, or you're breaking tournament rules. (Suffice to say, breaking those rules will get you a stern talking to from your hosts, and/or disqualificiation from the tournament.)
7.3 Card Use in Limited Tournaments
Cards must be received directly from tournament officials. This product must be new and previously unopened. Pro Tour, Grand Prix, World Magic Cup, and World Championship events may have had boosters opened in order to stamp them. Each player (or team) must be given exactly the same quantity and type of product as all other players participating in the tournament. For example, if one player receives three Return to Ravnica boosters for a booster draft, all other players must also receive three Return to Ravnica boosters.
Emphasis added. If you bought extras, they're just extras you'll use in your own time. Don't bring them into the tournament.
Buying a box of cards is not the same thing as buying entry into an event. If the store lets you buy spare boxes, that's cool, but when you play in the event it's still just the normal event.
From Wizards' page on prereleases:
Prereleases are Sealed Deck format events, where you build a minimum 40-card minimum deck using the contents of a themed Prerelease Pack.
That's a pack, not as many as you happen to own. To be very specific, you open a pack, and build from it. You do not open two and pick among them.