Snap Shot & Improved Snap Shot feat allow you to threaten up to 10 feet from you
Correct.
or 2 squares on combat.
Pathfinder is generally played on a grid representing 5-ft. squares, so two squares would be equivalent to 10 feet, yes. Distances aren’t really measured in squares, though, unlike some other systems (e.g. D&D 4e).
You can make an attack of opportunity to any hostile entering [...] that space.
No, or at least not unless you have something additional beyond what you have mentioned here. Attacks of opportunity are provoked only for leaving a threatened space, not for entering it.
You can make an attack of opportunity to any hostile [...] leaving that space.
Correct.
Combat Reflexes let you make multiple attacks of opportunity up to your Dex mod.
Actually, it’s up to your Dex mod plus 1. The feat specifies a number of additional attacks of opportunity up to your Dex mod, so that’s in addition to the one you always have.
Aside from that, correct. Note that Combat Reflexes also does other things.
If a hostile enters your 10-foot threaten area, you get an attack of opportunity.
As already addressed, false.
Regardless whether the attack hits or not, the hostile presses on to your 5-foot threatened area. Are you allowed then a secondary attack of opportunity at the same hostile for crossing both your threatened zones?
Even ignoring the bit about entering squares, and assuming a similar scenario but one in which the target exited multiple squares, false. You would only ever get one attack of opportunity for something like this.
Generally speaking, you only take one attack of opportunity for a given provocation. Moving out of a threatened area is a provocation, and they do provoke each time they leave such a square, but it’s all one provocation. You can only take the attack of opportunity for it once. You can choose not to take the attack of opportunity and then take it on a later square, if you like (though you risk the enemy not doing what you thought he would, and not provoke again). But the point is, you only ever get one attack of opportunity for it. Combat Reflexes just means that you could also take an attack of opportunity if someone else moved, or if the same person did something else that provoked, e.g. cast a spell, or used a ranged attack, or whatever.
This can get kind of complicated, for example, if someone casts a spell that makes a ranged (touch) attack, they provoke two attacks of opportunity from you, and you can actually take both. One provocation is casting a spell in your threatened area, the other is making a ranged attack in your threatened area.
But for movement, they only provoke again if they make some kind of separate movement. That usually (always? can’t think of any counter-examples off the top of my head) means a separate action.
Oh, and also, it doesn’t matter at all that the hostile moved from 10 feet away from you to 5 feet away from you. Leaving a threatened space is the only thing that matters. An enemy circling around you at a distance of 10 feet would also provoke twice (but again, you could only take the attack once).
