|Basically all users need to switch to another standard gateway.|
So assuming cable routers internal ip is 192.168.0.1.
Also assuming dsl routers internal ip is 192.168.0.2.
Then the clients only need to change the gateway and dns settings from 192.168.0.1 to 192.168.0.2, to switch from cable to dsl.
If you'd like to manage that, you need another router, where you use the WAN port configured with an internal static ip, e.g. 192.168.0.3 and as gateway the cable or dsl routers internal ip.
In this case all clients will use 192.168.0.3 as their standard gateway and you decide, when your users use which internet connection, cable or dsl, by changing the gateway at this router.
Please send a reply, if you solved the problem !!!