Ok, I release my new app and I want to boost downloads a lot. I already have 1 quite succesfull app, so I decided to use it for that purpose.
So question is: is creating notification in status bar (which will contain few words and link to my new app) legal or illegal? Can Google ban me for that?
No, you are not allowed to add notifications that link to another app.
Google can ban you for this. ILLEGAL
OK, but what if I add notification that after click on it will open my app(app that created this notification) and then show link to my another app(or for example AppWall)?
You can add a notification if it relates to your first app, but Google does allow that when you open your first app from that notification user will see an ad from you … just need to play it smart so the notification is “first app relevant”. Best if its a game, then you could give some bonus to users and announce it this way. For utility app that would be a little bit trickier.
Frankly, I believe that having this add as a “on open” one for your app will not give you much worse results than the notification based one 
I will probably create notification on which when user clicks simple dialog appears, where will be written something like “Special offer for all users - today you can download my other app for free (click on OK), otherway cancel” or something like that - then it should be ok, don’t you think?
That should be fine. Do consider two aditional points:
- If the app is free - users may feel being cheated … its not really that special of an offer
- Not all your users speak english - may be worth to make this multilingual.