This has changed over the years with 'cable select', but your master should be at the end of the cable and the slave should be in the middle.
It used to be the other way around, many years ago.
I would like to discourage the use of cable select though. Setting your drives as master/slave manually using jumpers is the most reliable method.
In most cases, the position of the master and slave drive on the cable, when manually set, is not a concern.