Routing / NAT of second backup link

I am looking through the forum for a example similar to what I want to do below

I am looking at setting up a back up connection, and using a routerOS device. ( I do not have a device yet)

(my concern is regarding the incoming connections and unusual behaviour as we would be doing combination of routing and NAT/Forwarding depending on link and am not sure whether the scenario I am proposing is would work, or what types of issues I will come across.

I have a couple of servers/devices that accept remote incoming connections.
They are presently on an DSL link with a subnet of public IP addresses.

The back up link I have been asked to setup is via a HSPDA (3G mobile/cell) modem,
and is only assigned 1 public (external) ipvaddress.

I would like to be able to direct (some combination of routing and Nat/Forwarding etc) incomming connections on specific ports of the backup
3G connection ipaddress to the servers public ipaddresses,
eg port 2001 will go to server 1 (primary public IP 1)
port 2002 will go to server 2 (primary public IP 2)

I am looking for comments and suggestions regarding this.

Thanks
shane

DIAGRAM (made up addresses)

            Primary                                                         Ba ckup
             ADSL                                                        HSPDA/3G 
              Link                                                            Link
        (public subnet)                                               (1 public IP)     
    (111.111.111.0/30)                                           (123.123.123.100)    
                |                                                                  |
                |                                                                  |
                |                                                                  |             
               +-----------------------   MicroTik  -------------------------+
                                    routes 111 subnet
            NAT 123 Address to 111 addresses depending on port 
                  eg 123.123.123.100 port 2001 to 111.111.111.1 port 2001
                 and 123.123.123.100 port 2002 to 111.111.111.2  port 2001
                                                 |
                                                 |
               +--------------------------------+-------------------------------+
                |                                                                  |             
           111.111.111.1                                            111.111.111.2