Hi there
The problem is not with your broadband connection at all as your uplink of 128k is more than enough to cope with up to 3 VoIP Adapters running (even if they used the G-711 aLaw Bandwidth)
The problem that you are experiencing is related to more your IP address - because each username is unique and on the GoTalk VoIP server when you log in with one username name, your public IP address(which comes from your ISP) is assigned to your username - this way round GoTalk knows how to locate you and your ATA sends out a live ping..
What you should do is when you connect your first ATA copy and paste the Diagnositc Log for that ATA and do the same for the second one and send it to GoTalk CSC - who do have some very helpful back office staff
Also ask GoTalk CSC to make sure that your account has been modified for MULTIPLE access
ALSO makes sure that the UDP PORTs are not the same one should be set at 5060 and the other should be set at 5061
Ciao
GoTalk