I have two Asterisk boxes which is connected to this same VoIP provider but with different credentials. When first box works then second cant’ receive any returning packet. When I stopped asterisk on first box then second have good communication with ISP and after that when I start again Asterisk on first then second box loose connection.
You will need different, but fixed, port numbers on the public network side of the NAT, and a service provider that is prepared to identify you by port number. As an example of the last point, from the other side, many people blindly put insecure=port into their configurations, which, if the roles were reveresed, would prevent two callers with the same public address being distinguished.
There will less to go wrong if you change the port number on one Asterisk, so that the port forwarding doens’t have to change the port number.