I have two lines in my 2830, and want to have both lines update two seperate dyndns domains.

Do set one to wan1 only, and one to wan2 only? But confused by the only/first options?

If the router reboots, I want both to update together. If one line drops, I want that line (or both) to update.

Thanks.

Check the guides online, there should be a wealth of info on how to configure DDNS. Also check out the draytek.com website, it too contains lots of info on how to do this.