new_log = Log()
db.session.add(new_log)
...
if some logic:
   db.session.rollback()
else:
   db.session.commit()

db = SQLAlchemy(app)
logger_db = SQLAlchemy(app)

new_log = Log()
logger_db.session.add(new_log)
logger_db.session.commit()
...
if some logic:
   db.session.rollback()
else:
   db.session.commit()