I painted 3 outdoor wood benches with interior semigloss latex over exterior latex. It has lasted for YEARS and the benches are outside all winter in the Northland where it's cold, wet, and snowy.
Just do in coat of exterior then. Every little bit helps. Save the other for inside.
Use an exterior primer.
NO. It will cause the exterior top coat to fail prematurely You MUST use proper exterior primer.
