Is there anyway to setup 2 dsl modems connected to a rb450 on ports 3+4 and 2 lan cables connected to ports 1+2,
use the following senario

lan 1 uses a pppoe username dialing out on port 3
lan 2 uses a pppoe username dialing out on port 4

basically what i want to do is anything from lan 1 must leave and return through the pppoe connection on port 3
and lan2 goes and returns on port 4

They do not need to see each other.

I got the dial up working on a single connection, however when i connect both pppoe-clients, they both get the same gateway ip address from the telecom provider, different ip address though.

How do i get this fixed?

I have attached the route list

If you have subnets on each port vs a birdge its simpler, but yes you can do this. A MT Consultant can help you with it. You can find a list of them at http://www.mikrotik.com/consultants.html