Well if you have two PCs connected to your router you do actually have a local network. Actually, being pedantic

, technically you also have a local network even if only one PC is connected to the router. Because the router is itself a network device that your PC is communicating or networking with. Thus, while you may not actually be using the router for PC to PC networking, but only using it to allow both PCs to access the Internet, the facility for PC to PC local networking is already there. At most, you might have to enable any firewalls you have installed to allow communication between the two PCs.
BTW, if you install Networx on all PCs that connect to the Internet through your router so that the program automatically monitors the total Internet usage of all those PCs, you will need to 'tell' each copy of Networx to communicate (i.e. network) with every other copy of Networx on other PCs connected to the router. This keeps all copies of Networx in sync across all your PCs and so accurately monitors the Internet usage of all your PCs.