Benjamin Moore claims their new Aura alkyd paints can be painted over darker colors in one coat; however, you must wait until the paint dries before you touch up if you miss a spot and it's absolutely critical to keep a wet edge. Practice with this on a piece of scrap substrate before you attempt to paint any interior or exterior surface if you are unfamiliar with the paint's drying characteristics. A stiffer brush is also recommended to reduce the number of brush strokes required to obtain the proper spread & thickness.
Since primers consist primarily of resins and a much lower solids content than paint, they are typically less viscous (thinner) and more spreadable without the requirements of keeping a wet edge. Their lower viscosity helps them penetrate microcracks better, so you will get better protection from a primer's ability to penetrate smaller openings in the substrate. This is a bigger issue with weathered exterior substrates, so a dedicated primer is probably the way to go for repainting exterior surfaces if it is determined that a primer is needed for repairs that penetrate to bare wood, for stain blocking, to improve bonding, or to extend the life of the topcoat(s). Most manufacturers recommend 2 topcoats regardless of whether you use a dedicated primer coat, especially for exterior surfaces exposed to the elements (temperature, humidity, dew point, freeze/thaw conditons) & UV. The exception is interior surfaces, which seldom have surface irregularities, have stable temperature & humidity conditions, and are not exposed to UV.