|You could do this one of two ways.|
1) have both LAN's use the same subnet
2) have the two LAN's use separate subnets.
#2 option would require a router with a route between the two subnets.
#1 would require you to manually setup the second LAN so as to avoid duplicate IP's
To my understanding, connecting a switch to the server and then branching out to both networks (via the switch) should not be done, as there can be conflicts between both the network's dsl lines
This is true with regard to the first scenario I listed above. But as I said, if you manually setup the LAN on the second router, you could easily avoid problem.
It's worth noting too that you don't necessarily have to use a switch between the two routers and then plug the server in to the switch. In either scenario you would leave the server plugged in to one router, and then you would plug the second router into the first router.
If you click on my name above in my response and then read my "how-to" guide on "adding a second router" you can see the two different ways to do this.
What you need to know is, if you used the option where all computers are in the same subnet, then they all have access to each other. If this is an issue, then you'd need to go with the separate subnet scenario.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.