You don't have to prime anything with a stain. Actually, using a alkyd stain prior to the latex will interfere with the latex stain and cause problems later.
The best way to deal with staining cedar siding is to wash it first with pressurized water, let it dry for a couple of days then apply the stain. Washing will remove dirt, dust, pollution and most of the old latex stain which is barley holding on after 20 years.
If you can't wash the house then just apply the latex stain. It is possible that the old stained side might look a little different on the first coat, but this will be taken care of with the second coat.
A better alternative to regular exterior latex stain is emulsified oil stain. It is a combination of alkyd and acrylic resins in a water base. Very good type of stain for Cedar.