Okay... some definitions are in order, because I don't think you're quite understanding what I mean by "rules-as-written" and "rules-as-intended".
Rules-as-written (RAW) is the letter of the law. It is the precise codification of the rules as laid out in the book.
Rules-as-intended (RAI) is the spirit of the law. It is the animus behind the rules, made evident by either a developer saying, "This is what this means" or by extrapolation based on the rest of the rules.
You keep talking about the RAW and acting as if it was the RAI. Numerous people have pointed out to you the plain intention of the rules (the RAI), but you keep going back to what is written (the RAW) to discount what seems, to most of us, to be obvious. That is either intellectually dishonest or personal naivete. If it is the latter, I apologize for suggesting you were insulting our intelligence. However, you seem to be a smart person, so I find it hard to believe you can't see the logic that's been laid out for you in this thread.
When you have rules-as-written that seem incomplete on a subject, sometimes you have to extrapolate what rules you do have in order to understand rules-as-intended. To that end, let's look at what rules we do have.
- In the description of Cybernetic Arms, we find that multiples of the same bonus from cybernetic arms don't stack.
- Such text was omitted from the description of the CBI.
- A Cybernetic Brain Implant, however, is described as a unit that sits over the ears and wraps around the back of the head, melding with the being's brain.
- Based on the description given (and the game item being modeled after Lobot's CBI from The Empire Strikes Back ), we can assume that there is no room on a being's body for more than one CBI. This is the likely reason for the lack of text referring to multiple CBIs stacking.
- While it's possible that there are other models, we can only assume they would look and function similarly to the model provided in the core book. Otherwise, we would also be able to assume there were Heavy Blaster Pistols capable of being stored in a coat pocket with ease, or Laminate armor that looked and acted like normal clothing.
- Fortunately, there is a system for upgrading items in Edge of the Empire : Mods.
- Unfortunately, the CBI has no hard points for mods, and can therefore not be upgraded.
- Further, the game was designed to limit the advancement of characteristics. There's no reason to assume the developers would be interested in providing an alternate advancement track for characteristics based on something as mundane as in-game money (which can be handed out by a GM like candy).
If you follow the rules-as-written in order to extrapolate rules-as-intended, the only logical conclusion is that no, the CBI doesn't stack with other CBIs, and for multiple reasons. I should emphasize that this shouldn't stop you from playing the game however you and your group wish. It's your game, have fun. But if you use the "I came up with my own model" excuse to do something the current items in the game can't do, just understand that you are creating a house rule, not playing according to the book.