You would by default need to connect the two devices together on the lan side yes, even to be able to access it without having to unplug the pc from the netgear and plug in to the thomson.

But without adding a second gateway it would have no route to get to the thomson IP.

When I was using a Thomson BE Box in bridge mode with my Draytek 2820 doing the routing I had port one of the Thomson conected to the WAN 2 input of the router and a second cable connected to port two (I think) of the Thomson connected to one of the four LAN outputs of the Draytek.

It then quite quite happily recognised this and 192.168.1.254 gave me the login screen of the Thomson (the Draytek runs as 192.168.1.3 for historical reasons).

The Thomson is a pile of **** so I ditched it and connected the Draytek diirectly to the ADSL again.

That's what I do with my netgear DG834 in modem mode.
Set it's LAN IP to be on the same subnet as your LAN.
So if your router is on 192.168.2.1 set the modem to 192.168.2.2 or similar.

One cable from the Netgear goes to the Routers WAN port and then a second cable goes to a LAN connection.

I can access the modem from any PC on the LAN this way.