Check out the website
http://fsxbeyondgamespy.com/ for information on many online servers that continue to operate through various server methods.
Gamespy was convenient for users such as yourself who were looking for a quick "no holds barred" multiplayer romp. But with it's demise, you are left seeking other ways to hook up with you sim pals to get it on in the skies.
I am only familiar with a couple of the direct connect servers, and I will say that they will not permit the activity you wish to partake in. It is their perogative however, and so it is to be respected. The other option is that one of you opens your ports properly, and creates a LAN session with the FSX multiplayer dialogue, and then tells the other player the IP address to connect to. I recommend that the person with the best internet connectin and the fastest computer serves as the host. Sorry, I am presuming you are using FSX. I know nothing of how to conenct with other versions of FS.
Hope that helps