You *CANNOT* simply split ethernet connections, you need to use a Network Hub/Switch.
When you refer to an ADSL CAT5 cable, that would suggest that you have a router and the main PC's Network Adapter connects to one of the router's LAN ports. If this is the case, why is the second computer not also connecting to the router? This would provide both computers with simultaneous internet access, as well as networking them for file and/or printer sharing.
If there is a particular reason why the main computer needs to connect to the internet via its ethernet port, and the second computer cannot connect at the same time, then you can *SHARE* the main PC's internet connection and continue to connect to the second PC with the "crossover" cable, *BUT* the main PC will need to have a second network adapter installed to do this.