Two internet connections, two routers, one network
Yes, disable DHCP on one of the routers, and give it a static IP address in the range that the other router uses.
Then connect the LAN ports of the two routers together.
This means that you now only have one internal network, and two ways to get to the internet. You can only have one default route, so only one of these routers can be used for "general" traffic at a time. So your options are to
- Manually switch the default gateway of some devices to use the other router as default gateway
- Set up static routes so some destinations are routed through the other router
Given that you are intending to dedicate one of the DSL lines to the NAS for downloads, this is straightforward. Give the NAS a static IP address, and make its default gateway the router on the dedicated line.