On any IDE port (you're concerned with the Primary here) you MUST have a Master device, and you MAY have a Slave. Usually these are set by placing a jumper on the correct pin pair on the back edge of the unit, according to the diagram on the drive itself. So, do you have a Master drive already? And this Slave you speak of, is its jumper set to Slave?
Once the jumpers are set, you SHOULD plug the one set to Master into the END connector of the data cable, and the Slave into the middle connector. Of course, both drives will need power connections in addition.
The alternative is to set the jumpers on BOTH drive units to "CS" (Cable Select). Then the one plugged into the END will be the Master, and the middle one the Slave. BUT you cannot mix the systems - either set Master and Slave, OR set BOTH to CS.