What (I think) you're describing is a site-to-site routed WAN topology that's common for connecting branch offices together, but almost always uses some form of VPN, most common being IPSec.
Encryption does add some overhead, but there's really no other way of getting around the insecurity of a public Internet connection without the benefit of encryption and authentication that a VPN provides for you.
You can talk to your ISP and have them setup LAN extensions for you, but they're going to be much more than your ADSL connections if price is a concern and may not be available as an option.
I say give the VPN a try: if your router/firewalls are not VPN capable, you can setup asynchronous OpenVPN tunnels on a spare workstation running Linux/BSD, etc. at both locations; these new OpenVPN servers will maintain the VPN tunnel and route traffic between both locations.
You'll have to setup static routes at each locations' default gateway (ADSL router, hopefully you can do that) to route traffic destined for the other location's local subnet to the local OpenVPN server (which is acting as a router now).
See this section at OpenVPN for more information:
Playing the guessing game here, but after re-reading your question, it sounds like you want the 10.0.0.0/24(?) network to span across WAN connections and the only way to do this would be to bridge it. OpenVPN can do this, but I don't think that's what you want, especially if bandwidth is a concern as you'd be on one big broadcast domain. Much better idea would be to route only the traffic you need across the tunnel.