I'm not overly familiar with DSL, but it shouldn't be necessary, and my guess is that -- if both are gateways (combination modem/routers) and he's using wifi, he's feeding off the signal from the primary devices and the second device is just an ornament; in normal operation the secondary one shouldn't be able to do its job hooked through a router. It's also possible to use the primary as a bridge and the latter strictly as the router, if desired, but it doesn't sound like he's likely to have configured it such.
Indeed. The BT engineer left it like that after installation. If it was my connection I'd be looking into it, but this is just academic curiosity. When I'm ultra bored one day I'll look it up to see what's what.

