lammert - 5:01 pm on Apr 29, 2009 (gmt 0)

Yes you can do this, but you have register all four dns servers for your domain name. There are no duplicate penalties involved if both the site at webhost 1 and 2 have the same domain name. Search engines see them as one site on multiple IP addresses. If host 1 goes down, visitors may still have load problems because in 50% of the case the DNS server will return the IP address at webhost 1, even if that server is unavailable.

An alternative is to use a third party DNS provider. You normally point the A record of the domain to the server at webhost 1. If that server goes down, you point the A record to webhost 2. If you use a small TTL (time to live) on the A record, the switch will be quite fast and visitors will only face downtime during a small period.