I don’t quite understand, how you can get 1.5Mbps, if you both have the same connection type. Download for one is upload for the other, so there will always be a bottleneck somewhere at 600Kbps. 400Kbps sounds reasonable with a 600Kbps connection, you will never get the full theoretical speed. BUT 70Kpbs is too little. However, there is a known bug in Cerberus, that could be causing this. (Known meaning the effect is known, but the bug itself has not been identified yet.) Have you tried to switch on “Optimized file sending”? (Server administration -> Advanced) That might also be the reason.
Don’t expect any more than around 400Kpbs though – unless I have misunderstood the description of your connections.