There are several ways to handle this, the easiest of which is to use the same IP address range with each ISP. This eliminates any DNS concerns.
If this isn't possible, you can give each webserver two IP addresses (one from each ISP) and have DNS return both of them in response to queries. Most/all web browsers will eventually try the second IP (within a minute when I tested this about a year ago) if the first is unresponsive.
Another option is some type of web failover product such as Cisco CSS or F5 3DNS. Such products can monitor web servers for uptime and adjust their DNS responses accordingly when an outage is detected.
If you use 2 different IP ranges but you run your own DNS servers, you could also put one DNS server in range1 and put another in range2. Then configure the first DNS server to issue only range1 addresses and the second one to only hand out range2 addresses.
The TLD servers will make sure the requests to your DNS servers are loadbalanced, and hence the requests to the servers will be distributed.
You would have to keep the DNS TTL low to prevent caching problems. The lower the TTL the more evenly balanced the traffic should be (but the higher the load on your DNS servers).
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