As stated, it all depends on the router's capabilities. Some only have basic failover (where only one connection can be active at a time), others have active load balancing combined with sophisticated policy routing that will quite literally let you do whatever you want.

So let's look at your needs.

It sounds like he has low latency, low bandwidth DSL for 4Sight, and you want to add high latency, high bandwidth satellite for movie streaming.

4Sight is an "incoming" service that he will be remotely accessing correct?

If you're using a router-based DDNS service for that, then so long as you have "active" load balancing (and not cheap failover), your specific dual WAN setup shouldn't matter too much, because DDNS will only ever bind to one routable IP address at a time, and you should be able to specify it use the DSL connection regardless of whichever WAN connection is set as "primary" (with the option of failing back to the other if you wish).

But, let's look a little further at what's possible with an advanced router.

* You can policy route specific internal IP addresses in the house to ONLY use a specific WAN connection (or in what order).

* You can policy route specific outgoing IP addresses or ports to ONLY use a specific WAN connection (or in what order).

* You can have it so that if one connection is saturated (typically upstream is monitored, but more sophisticated routers will also do downstream), it starts using an alternate WAN.

* You can policy route to use different WAN connections by time of day or day of week.

There's more, but those are what seem applicable to your situation. However, it can take a bit of work to figure out how to set up these advanced features on advanced routers (I know from experience - the router I have now will do all of that), so this might not be something you want to tackle, especially since any minor changes to his situation will require your involvement.