Django when a data is entered, write the data in the other model depending on the condition

Question

Hello There is a data(text) in data b on app1. I want to write the data here on d on app2. how can I do that?

app.model;
class Post(models.Model):
    a=models.CharField()
    b=models.TextField()

app2.model;
class Post2(models.Model):
    c=models.CharField()
    d=models.TextField()