Multiple external IP + user key

[Alex V]

Our company have two internet connections, each with permanent IP. I want some user being able to connect to the SOS server using any of lines (both are not reliable).
Inside SourceOffSite Server Manager I create a key for the <first IP>/<user name>. Send resulting key file to the user, he is able to connect. So far so good.
When I try to create the second key for the <second IP>/<user name> I get a message that "<user name> already has a key".

Is there a way to let user to connect to any of two IP addresses, whichever is available at the moment?

[lbauer]

Is it possible for the user to connect with the name of the server: sosserver.yourdomain.com?

[Alex V]

No, there are no DNS names for our external IP addresses.