The phrase in to and the word into are not interchangeable. In fact, they have totally different meanings and you're supposed to use them different ways. Honestly, if you use them the wrong way most people aren't going to notice. But if you are on a quest for grammar perfection, you're in the right place. Because I know what the differences between them are.
Not That Into You
Grammatically, saying he's just not that into you is not correct. It should always be in to you. But how do you know the difference?
To get technical about it, into is a preposition. When used in a sentence, it indicates movement. Something is happening when into comes into play. In and to, by contrast, are used to indicate position. It's not quite as confusing as it sounds right now.
Into is for movement. Remember that, and it begins to get easier. Let's look at some examples:
She pushed the peg into place.
He reached into the jar.
Harry moved into the bedroom.
When there is no movement in the sentence, you're probably supposed to be using in and to instead. For example:
She saw in to the future, and it's bad.
The peanuts are baked in to the cake.
I said don't look in to the tank.
In and to are used when in is interacting with another word in the sentence. If you looked in or sang in or broke in, the in is attached to that verb. That means it's not with to, and to is just another word in the sentence. And pay attention to the sentences: there's no movement.
Remember that into is in to itself; it makes itself move. In to can't stand alone; it's got to attach to a verb. Stick to the basic rules, and it's much easier to use the right words in the right places.