How to set default gateway for two network cards

I've the following situation:
One server two networkcards. One is connected to the outside world to retrieve data via port 3000. The other is connected to the internal network, to let people connect to the database on the server.
Sometimes the proces outside, that should connect to this server via port 3000 can't see the server, because the server uses the internet connection that is used via the internal networkcard. So, when i run www.canyouseeme.org, i connects via the internal networkcard and not via the one that is connected to the outside world. When i turn of the internal card, it goes well, at that moment www.canyouseeme.org uses the external card.
How can i connect this server so, that people from the inside network can see the database on the server, and the process from the outside, can access the same server via the other network card. With other words, the internet connection should always and only go via the external network card, only communication to internal clients (to acces the database should go via the other network card)