Sorry no one has been able to answer and I don't really know MultiWAN.

In your first post, were you asking which WAN interface is used for the ClearOS Dynamic DNS? If so I don't know the answer but perhaps someone else can post. Otherwise, expecially if your dynamic interface (either PPPoE or Ethernet) directly gets a public IP you can use ddclient on the server and a free Dynamic DNS provider such as DtDNS.com to keep an FQDN associated with your dynamic IP.

Another DNS point with MultiWAN. If your ISP's are different on the two interfaces, generally you need to configure ClearOS to use a Public DNS service such as OpenDNS or GoogleDNS rather than use your ISP's servers.

I don't know about line balancing but it depends on what you are trying to achieve.

If we Connect 2 ethernets combined what is the global DNS to give and eth1 is 5 mbps line where as eth2 is 100 mbps line
and also what is the weight to balance in multi Lan
please post your valuable suggestions