I think you're asking about what it would take for you to utilize multiple connections to the Internet for fail-over.
Fortunately, your Active Directory domain and internal DNS has little to do with such a scenario. Microsoft ISA Server doesn't natively support using multiple WAN interfaces simultaneously.
One solution would be to deploy a "multi-WAN" or "dual WAN" router in front of the ISA server. These are devices that perform rudimentary load-balancing and failover using multiple Internet connections. There are a variety of price-points and feature sets, so you'd do well to shop around and compare reviews.
You'll never get the full bandwidth of both Internet connections to be "shared" for a single TCP connection (a single download, etc) w/o cooperation from the ISP on the other end. W/ consumer / prosumer "multi-WAN" routers the best you can hope for is that the router would try and intelligently send new outgoing TCP requests over the less-congested link.
BTW: I've seen a couple reviews of these Peplink Balance routers just doing a rudimentary search. They're not inexpensive, but they appear to be fairly solid.
If you want to take a different approach you could cobble together some rudimentary failover using a script (like the one in this discussion) to test the availabiliy of a connection periodically and, if necessary, swap the default gateway to another connection. I suppose it would work, but I'd rather have such functionality happening in hardware.
There was a product called "RainConnect" that had both inbound and outbound failover and load balancing capability on ISA Server, but the product has been discontinued, to my knowledge (when the manufacturer was acquired by EMC).