If you've got a spare PC then fit that with 3 NIC's. Then plug one to the ADSL and one to the SACM, this is the WAN side. The other is the LANside to your PC. Now install something that can do the routing for you. There is probably as Linux distro already available for that purpose that can also prevent access between the 2 WAN ports.
One issue is what networks the 2 modem devices want on their LAN side as it is often very similar. They probably want to use 192.168.x.0/24 and hopefully the x is different. If you are using a router for the ADSL side then that may be configured to something else, similarly on the cable side.
You are then aiming for something like
PC ---- router PC ------ (router) ---- SACM
\----- ADSL router
If you put a firewall on the router PC you can build in a proper DMZ if the routers have more than 1 LAN port on too. Or add a forth NIC to the router PC.
PC -------- router PC -------- ADSL router ------- BT
| \-------- Cable router ------- VM
Put a switch where your PC is and then you can have multiple devices
sharing the setup.