My point was that I had two SIP endpoints registered at the same time, through the same Internet connection. So if that doesn't work for you that's probably not because of some inherent limit in SIP, but maybe something to do with your firewall or maybe the SIP service. Does your Fritz box original its SIP from your inside LAN, and then apply NAT, or does it "grab" the outside IP address (so to speak)? It's probably also a scenario where SIP inspection/ALG would be harmful, on hosted systems we've found only one phone can register until the ALG is disabled.
My test was using two devices on the inside LAN both going through the same NAT process to the same service, NAT done by a Zyxel router with ALG disabled.
I agree about random softphones, I mention that Micro SIP only for test purposes, and a portable application that doesn't actually install anything into Windows reduces any risk.