Let's assume we have device which allows entering the same credentials on the both lines.
This should not be a problem if this device has public IP, i.e. no NAT involved.
If we have SIP device behind NAT then only one line will take incoming call because NAT device will have only 1 single mapping between {internal address:port} and {xx.voxalot.com:port}
Probably using different ports on voxalot side will solve a problem. In this case router will have something like this in the NAT table:
192.168.1.10:5060 -- WAN_IP:5060 -- xx.voxalot.com:5060
192.168.1.10:5062 -- WAN_IP:5062 -- xx.voxalot.com:4060
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