Both toilets won't flush and when you run any water it comes into the master bathroom shower and it will not drain

Let's take it one bathroom at a time, Do both bathrooms do the same thing? When drained or flusher the both come up in the shower correct? You have a blockage downstream from the shower that must be snaked out. You can snake from any one of three places.
1- Pull the toilet and snake from there,
2- Snake from the shower drain. **OR**
3- You can snake from the lavatory roof vent, Put out enough snake to reach the base and 15 more feet. Good luck, Tom