Consider using a hub - brings a lot of flexibility to the table and will solve you current problem plus LAN much easier to set up and a lot less tweeking
I abandoned the cat5 crossover thing some time ago for exactly the problem you are experiencing and will never go back - I can have both Pc's on line both with there own modems and transferring data peer to peer at the same time, another setup used one PC as a gateway so could likewise have both PC's on line and still be doing peer to peer sharing.
Old dogs CAN learn new tricks
Good Judgement comes from Experience - Experience comes from Bad Judgement - but bad judgement is more fun!!