Is it possible to make chocolate with cocoa butter and cocoa powder or any two other ingredients only?
As discussed in the answer to Making dark chocolate at home (and additional detail on  Is it possible to sweeten chocolate without making it gritty?), making chocolate at home is basically not feasible. You need specialized equipment, because chocolate is not just a mixture of cocoa and other things, it's been ground and mixed in an extremely time-consuming and specific way.

Setting that rather large issue aside...

Sure, chocolate can be made with only cocoa solids and cocoa butter. It's called unsweetened chocolate, and people usually only use it for baking, since it's pretty bitter on its own. The core ingredients are normally cocoa (solids/butter) and sugar. After that, it's mostly about flavor: milk for milk chocolate, salt, maybe even vanilla, and so on. All that can be omitted, but without the sugar, it won't be what most people think of as chocolate for eating.

The percentage is the amount that's from cocoa, whether solids or butter. Unsweetened chocolate with nothing added is 100%, and 70% dark is likely close to 30% sugar (maybe a bit less if other things have been added). I believe that unsweetened chocolate is usually something like 50-55% cocoa butter and 45-50% solids, so I guess that's about the maximum to retain a normal texture if there's nothing else going on.

But again, unless you've got some serious chocolate manufacturing investment plans, all you can do with that information at home is use it to pick what you buy.