On the second machine, you can add the server PC as a license source in the FactoryTalk control screen by adding a license directory, but you use the name for the 'directory'. With a concurrent license, when you activate the license on a PC, that PC is a server. If you don't specify/buy the concurrent one up front, you get node locked. There are two types of licenses: Concurrent, and node locked.
There are also countless not so legal methods which is the reason that Rockwell went to the FTA (FactoryTalk Activation) scheme. If you are using the old EVRSI licensing scheme, the only practical (but pain in the rear) way to legally do it is to juggle the license back and forth using EVMOVE and/or carrying the floppy master disk around with you.
